title: Guinness World Records

text: Traditionally, the company made a large amount of its revenue via book sales to interested readers, especially children. The rise of the Internet began to cut into book sales in the 2000s and forward, part of a general decline in the book industry. According to a 2017 story by Planet Money of NPR, Guinness began to realise that a lucrative new revenue source to replace falling book sales was the would-be record-holders themselves. While any person can theoretically send in a record to be verified for free, the approval process is slow. Would-be record breakers that paid fees ranging from US$12,000 to US$500,000 would be given advisors, adjudicators, help in finding good records to break as well as suggestions for how to do it, prompt service, and so on. In particular, corporations and celebrities seeking a publicity stunt to launch a new product or draw attention to themselves began to hire Guinness World Records, paying them for finding a record to break or to create a new category just for them.


title: Guinness World Records

text: "Guinness World Records" was criticised by television talk show host John Oliver on the program "Last Week Tonight with John Oliver" in August 2019. Oliver pointed serious criticism at Guinness for taking money from authoritarian governments for pointless vanity projects as it related to the main focus of his story, President of Turkmenistan Gurbanguly Berdimuhamedow. Oliver asked for Guinness to work with "Last Week Tonight" to adjudicate a record for "Largest cake featuring a picture of someone falling off a horse," but according to Oliver, the offer did not work out after Guinness insisted on a non-disparagement clause. "Guinness World Records" denied the accusations and stated that they declined Oliver's offer to participate because "it was merely an opportunity to mock one of our record-holders," and that Oliver did not specifically request the record for the largest marble cake. As of 2021, the Guinness World Record for "Largest marble cake" remains with Betty Crocker Middle East, set in Saudi Arabia. Following Oliver's episode, Guinness World Records' ethics were subsequently called into question by human rights groups.


title: Guinness World Records

text: In 1976, a "Guinness Book of World Records" museum opened in the Empire State Building. Speed shooter Bob Munden then went on tour promoting "The Guinness Book of World Records" by performing his record fast draws with a standard weight single-action revolver from a Western movie-type holster. His fastest time for a draw was 0.02 seconds. Among exhibits were life-size statues of the world's tallest man, Robert Wadlow, and world's largest earthworm, an X-ray photo of a sword swallower, repeated lightning strike victim Roy Sullivan's hat complete with lightning holes and a pair of gem-studded golf shoes on sale for $6,500. The museum closed in 1995.


title: Guinness World Records

text: In more recent years, the Guinness company has permitted the franchising of small museums with displays based on the book, all currently () located in towns popular with tourists: Tokyo, Copenhagen, San Antonio.


title: Guinness World Records

text: There were once Guinness World Records museums and exhibitions at the London Trocadero, Bangalore, San Francisco, Myrtle Beach, Orlando, Atlantic City, New Jersey, and Las Vegas, Nevada. The Orlando museum, which closed in 2002, was branded "The Guinness Records Experience"; the Hollywood, Niagara Falls, Copenhagen, and Gatlinburg, Tennessee museums also previously featured this branding.


title: Guinness World Records

text: Guinness World Records has commissioned various television series documenting world record breaking attempts, including:


title: Guinness World Records

text: With the popularity of reality television, Guinness World Records began to market itself as the originator of the television genre, with slogans such as "we wrote the book on Reality TV".


title: Guinness World Records

text: In 2008, Guinness World Records released its gamer's edition, a branch that keeps records for popular video game high scores, codes and feats in association with Twin Galaxies. The Gamer's Edition contains 258 pages, over 1,236 video game related world records and four interviews including one with Twin Galaxies founder Walter Day. The most recent edition is the "Guinness World Records Gamer's Edition 2020," which was released 5 September 2019.


title: Guinness World Records

text: "The Guinness Book of British Hit Singles" was a music reference book first published in 1977. It was compiled by BBC Radio 1 DJs Paul Gambaccini and Mike Read with brothers Tim Rice and Jonathan Rice. It was the first in a number of music reference books that were to be published by Guinness Publishing with sister publication "The Guinness Book of British Hit Albums" coming in 1983. After being sold to Hit Entertainment, the data concerning the Official Chart Company's singles and albums charts were combined under the title "British Hit Singles & Albums", with Hit Entertainment publishing the book from 2003 to 2006 (under the Guinness World Records brand). After Guinness World Records was sold to The Jim Pattison Group, it was effectively replaced by a series of books published by Ebury Publishing/Random House with the "Virgin Book of British Hit Singles" first being published in 2007 and with a "Hit Albums" book following two years later.


title: Guinness World Records

text: In 1975, Parker Brothers marketed a board game, "The Guinness Game of World Records", based on the book. Players compete by setting and breaking records for activities such as the longest streak of rolling dice before rolling doubles, stacking plastic pieces, and bouncing a ball off alternating sides of a card, as well as answering trivia questions based on the listings in the "Guinness Book of World Records".


title: Guinness World Records

text: A video game, "", was developed by TT Fusion and released for Nintendo DS, Wii and iOS in November 2008.


title: Guinness World Records

text: In 2012, Warner Bros. announced the development of a live-action film version of "Guinness World Records" with Daniel Chun as scriptwriter. The film version will apparently use the heroic achievements of record holders as the basis for a narrative that should have global appeal.


title: Byju's

text: Byju's (stylised as BYJU'S) is an Indian multinational educational technology company, headquartered in Bangalore, Karnataka, India. It was founded in 2011 by Byju Raveendran and Divya Gokulnath. , Byju's is valued at billion and the company claims to have over 115 million registered students.


title: Byju's

text: Byju's app was developed by Think and Learn Pvt. Ltd, a company which was established by Byju Raveendran, Divya Gokulnath and a group of students in 2011. Byju, an engineer by profession was coaching students in mathematics since 2006. In initial days the company focused on offering online video-based learning programs for the K-12 segment and for competitive exams. In 2012, the firm entered Deloitte Technology Fast 50 India and Deloitte Technology Fast 500 Asia Pacific ratings and has been present there ever since.


title: Byju's

text: In August 2015, the firm launched Byju's: The Learning App. In 2017, they launched Byju's Math App for kids and Byju's Parent Connect app. By 2018, it had 15 million users out of which 900,000 were paid users at that time. In the same year, Byju's became India’s first edtech unicorn. By 2019, 60% of BYJU’S students were from non-metros and rural cities.


title: Byju's

text: In January 2022, the company joined Simplilearn, Unacademy, upGrad, PrepInsta Prime and Vedantu to become one of the founding members of IAMAI's India EdTech Consortium.


title: Byju's

text: In March 2022, it signed a contract with Qatar Investment Authority to establish a new edtech company and a R&D centre in Doha.


title: Byju's

text: In August 2022, "Bloomberg News" reported that the Ministry of Corporate Affairs sent a letter to Byju's asking them to explain the non-filing of its audited financials for the year ending March 2021. Having missed the deadline by over 17 months, Byju's reasoned the delay was due to the challenge of consolidating the accounts of its acquisitions during that year. In spite of the delay, Deloitte gave the company a clean audit. In November 2022, many employees of Byju's came out to speak against the company's unfair treatment. Reportedly, Byju's sacked over 5,000 employees.


title: Byju's

text: Byju's is an education tutoring app that runs on a freemium model, with free access to content limited for 15 days after the registration. It was launched in August 2015, offering educational content for students from classes 4 to 12 and in 2019 an early learning program has started for classes 1 to 3. It also trains students for examinations in India such as IIT-JEE, NEET, CAT, IAS, and international examinations such as GRE and GMAT.


title: Byju's

text: Academic subjects and concepts are explained with 12-20 minute digital animation videos and through which students learn in a self-paced mode. Byju's reports to have 40 million users overall, 3 million annual paid subscribers and an annual retention rate of about 85%. In October 2018, the app expanded to the United Kingdom, United States and other English-speaking countries.


title: Byju's

text: In 2019, the company announced that it would launch its app in regional Indian languages. It also planned to launch an international version of the app for English-speaking students in other countries. And to cater students of kindergarten, Byju's launched new programs in its Early Learn App.


title: Byju's

text: In April 2021, the company also announced the launch of "Byju's Future School" to be led by WhiteHat Jr Founder Karan Bajaj. The Future School aims to cross the bridge from passive to active learning with an interactive learning platform blended with coding and other subjects like Math, Science, English, Music and Fine arts through storytelling. Byju's will launch the Future School in the United States, United Kingdom, Australia, Brazil, Indonesia and Mexico in May. Karan Bajaj quit WhitHat Jr a year after the acquisition.


title: Byju's

text: The company is focusing on adopting a hybrid model of teaching and learning by launching 500 tuition centers across 200 cities in India. As of February 2022, 80 centers are already launched.


title: Byju's

text: In July 2017, Think and Learn acquired TutorVista (including Edurite) from Pearson. In January 2019, Byju's acquired American-based Osmo, a maker of educational games for children aged 3–8 years for . Byju's also acquired Indian startup WhiteHat Jr for . In September 2020, Byju's acquires virtual labs simulation startup LabInApp. In February 2021, Byju's acquires Mumbai-based doubt clearing platform Scholr.


title: Byju's

text: In April 2021, Byju's acquired test prep firm Aakash Educational Services Ltd. in an estimated cash and stock deal. Aakash's founders and Blackstone Group will receive minority stakes in Byju's as part of the deal. In July 2021, Byju's acquired American-based kids learning platform Epic! in a cash-and-stock deal. The Epic acquisition was part of Byju's foray into the overseas market, from where it expects annual revenue of per financial year. In July 2021, Byju's acquired Singapore-based higher education platform Great Learning at a cost of and after-school learning app Toppr. In September 2021, Byju's acquired two startups including online test preparation platform Gradeup for an undisclosed price and rebranded it to BYJU’S Exam Prep and American based coding platform Tynker for . In December 2021, Byju’s acquired GeoGebra in a cash-and-stock deal.


title: Byju's

text: Till date, Byju's invested at least $2.8 billion on a dozen acquisitions in an apparent attempt to expand beyond the original learning app and thread together its services that will allow it to reach learners of all ages. For example, it has ventured into exam preparation, higher education MOOCs, and tuition centers. In addition, it has acquired several additional platforms that offer virtual reality, artificial intelligence, and educational gaming services, most of which occurred in 2021.


title: Byju's

text: Byju's received seed funding from Aarin Capital in 2013. As of 2019, Byju's had secured nearly $785 million in funding from investors, including Sequoia Capital India, Chan Zuckerberg Initiative (CZI), Tencent, Sofina, Lightspeed Venture Partners, Qatar Investment Authority, Verlinvest, IFC, Napsters Ventures, CPPIB and General Atlantic. Byju's was the first company in Asia to receive an investment from Chan-Zuckerberg Initiative (co-funded by Meta Platforms co founder, chairman and CEO Mark Zuckerberg and Priscilla Chan). As per the company filings with the Ministry of Corporate Affairs, Byju's became a unicorn and was valued at ₹6,505 crore ($1 billion) as of March 2018. In June 2020, Byju's attained the "decacorn" status with an investment by Mary Meeker's Bond Capital. In March 2022 Byju's raised $800 million, reaching a valuation of $22 billion.


title: Byju's

text: In September 2020, Byju's replaced Oppo as the title sponsor of the India national cricket team. Byju's operates roughly on a premium business model where a paid subscription is required for most of the content. In 2017, Byju's generated revenues of about ₹260 crore (US$40 million or €33 million) and doubled it in 2018 financial year, earning ₹520 crore. In June 2020, with the investment of Bond, a global technology investment firm, Byju's became decacorn at US$10.5 billion valuations.


title: Byju's

text: In November 2020, Byju's became the title sponsor of the Indian Super League club Kerala Blasters FC replacing Muthoot Group. In November 2020, Byju's raised US$200 million in a fresh funding round led by BlackRock and T. Rowe Price at a valuation of $12 billion. In March 2021, Byju's secured $460m in a series F funding round. In April 2021, B Capital, Baron Funds, and XN invested $1 billion in Byju's. In June 2021, Byju's raised $50 million in a Series F round from IIFL's private equity fund and Maitri Edtech. In October 2021, Byju's raised $296 million as a part of its Series F round from Oxshott Venture Fund, Edelweiss Group, Verition Fund, XN Exponent Holdings, and MarketX Ventures. In March 2022, the company raised $800 million from Byju Raveendran, Sumeru Ventures, Vitruvian Partners, and BlackRock.


title: Byju's

text: In September 2020, Byju's launched the “Education for All” Initiative for kids from marginalized populations.


title: Byju's

text: Among the several initiatives launched under Education for All, Byju's Give was launched in November 2020, under which the company collects old or unused smart devices for refurbishing purposes and then loads them up with Byju's content for free and distribute to those children who have no access to the internet.


title: Byju's

text: In November 2020, Byju's replaced the Muthoot Group to become the title sponsor of the Indian Super League club Kerala Blasters FC.


title: Byju's

text: Byju's was criticized for charging fees that only the rich can afford. Former salespeople say the company pushes its products on parents who cannot afford them. Byju's filed a crore defamation suit against critic Pradeep Poonia. They later withdrew the lawsuit.


title: Byju's

text: Byju's subsidiary WhiteHat Jr. was asked to remove their five TV advertisements by the Advertising Standards Council of India (ASCI) due to misleading advertisements and hard sales tactics. WhiteHat Jr. claimed that a child named "Wolf Gupta" bagged job offers worth crores in multiple social media advertisements. This child was often described to be between the ages of 6 to 14 years. Investigations later revealed that this was all fabricated and Mr."Gupta" is a work of fiction.


title: Byju's

text: The company had filed a defamation suit against Pradeep Punia in November 2020, a software engineer, who alleged that the company made false claims and hired incompetent teachers. It was later withdrawn. It has also been the subject of a data leak, where the personal information of over 200,000 users was exposed.


title: Byju's

text: The Department of Consumer Affairs voiced concerns on June 24, 2022, at India Edtech Consortium (IEC) meeting regarding aggressive sales practices and deceptive marketing strategies utilized by EdTech companies, particularly Byju's and the entities that make up its group. DCA advised Byju's to work closely with the ASCI based on the complaints against it. According to the reports, Byju's team has pledged to resolve these grievances with a detailed action plan.


title: Byju's

text: Previously, BYJU's used a different revenue recognition practice, which they had to change and obtain approval in accordance with Indian Accounting Standards (Ind-AS) 115 guidelines. For example, revenue expected in one fiscal year for a multi-year charge cannot be considered as revenue in that year alone. This discrepancy was eventually flagged by the auditor and a change was sought by the latter. As of September 2022, BYJU's began recognising streaming revenue across the term of usage, which was previously recognised fully upon the start of the contract. These new accounting standards have resulted in a reduction in the amount of income that Byju's is able to book in advance.


title: Byju's

text: Also, in the past, BYJU's used its funding capital to acquire both domestic and overseas businesses and group them under its brand. And, lately investors voiced scepticism regarding this very plan to rapidly deploy capital in order to grow internationally.


title: Bacteria

text: Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the first life forms to appear on Earth, and are present in most of its habitats. Bacteria inhabit soil, water, acidic hot springs, radioactive waste, and the deep biosphere of Earth's crust. Bacteria are vital in many stages of the nutrient cycle by recycling nutrients such as the fixation of nitrogen from the atmosphere. The nutrient cycle includes the decomposition of dead bodies; bacteria are responsible for the putrefaction stage in this process. In the biological communities surrounding hydrothermal vents and cold seeps, extremophile bacteria provide the nutrients needed to sustain life by converting dissolved compounds, such as hydrogen sulphide and methane, to energy. Bacteria also live in symbiotic and parasitic relationships with plants and animals. Most bacteria have not been characterised and there are many species that cannot be grown in the laboratory. The study of bacteria is known as bacteriology, a branch of microbiology.


title: Bacteria

text: Humans and most other animals carry millions of bacteria. Most are in the gut, and there are many on the skin. Most of the bacteria in and on the body are harmless or rendered so by the protective effects of the immune system, and many are beneficial, particularly the ones in the gut. However, several species of bacteria are pathogenic and cause infectious diseases, including cholera, syphilis, anthrax, leprosy, tuberculosis, tetanus and bubonic plague. The most common fatal bacterial diseases are respiratory infections. Antibiotics are used to treat bacterial infections and are also used in farming, making antibiotic resistance a growing problem. Bacteria are important in sewage treatment and the breakdown of oil spills, the production of cheese and yogurt through fermentation, the recovery of gold, palladium, copper and other metals in the mining sector, as well as in biotechnology, and the manufacture of antibiotics and other chemicals.


title: Bacteria

text: Once regarded as plants constituting the class "Schizomycetes" ("fission fungi"), bacteria are now classified as prokaryotes. Unlike cells of animals and other eukaryotes, bacterial cells do not contain a nucleus and rarely harbour membrane-bound organelles. Although the term "bacteria" traditionally included all prokaryotes, the scientific classification changed after the discovery in the 1990s that prokaryotes consist of two very different groups of organisms that evolved from an ancient common ancestor. These evolutionary domains are called Bacteria and Archaea.


title: Bacteria

text: The word "bacteria" is the plural of the New Latin ', which is the latinisation of the Ancient Greek ('), the diminutive of (""), meaning "staff, cane", because the first ones to be discovered were rod-shaped.


title: Bacteria

text: The ancestors of bacteria were unicellular microorganisms that were the first forms of life to appear on Earth, about 4 billion years ago. For about 3 billion years, most organisms were microscopic, and bacteria and archaea were the dominant forms of life. Although bacterial fossils exist, such as stromatolites, their lack of distinctive morphology prevents them from being used to examine the history of bacterial evolution, or to date the time of origin of a particular bacterial species. However, gene sequences can be used to reconstruct the bacterial phylogeny, and these studies indicate that bacteria diverged first from the archaeal/eukaryotic lineage. The most recent common ancestor of bacteria and archaea was probably a hyperthermophile that lived about 2.5 billion–3.2 billion years ago. The earliest life on land may have been bacteria some 3.22 billion years ago.


title: Bacteria

text: Bacteria were also involved in the second great evolutionary divergence, that of the archaea and eukaryotes. Here, eukaryotes resulted from the entering of ancient bacteria into endosymbiotic associations with the ancestors of eukaryotic cells, which were themselves possibly related to the Archaea. This involved the engulfment by proto-eukaryotic cells of alphaproteobacterial symbionts to form either mitochondria or hydrogenosomes, which are still found in all known Eukarya (sometimes in highly reduced form, e.g. in ancient "amitochondrial" protozoa). Later, some eukaryotes that already contained mitochondria also engulfed cyanobacteria-like organisms, leading to the formation of chloroplasts in algae and plants. This is known as primary endosymbiosis.


title: Bacteria

text: Bacteria are ubiquitous, living in every possible habitat on the planet including soil, underwater, deep in Earth's crust and even such extreme environments as acidic hot springs and radioactive waste. There are approximately 2×10 bacteria on Earth, forming a biomass that is only exceeded by plants. They are abundant in lakes and oceans, in arctic ice, and geothermal springs where they provide the nutrients needed to sustain life by converting dissolved compounds, such as hydrogen sulphide and methane, to energy. They live on and in plants and animals. Most do not cause diseases, are beneficial to their environments, and are essential for life. The soil is a rich source of bacteria and a few grams contain around a thousand million of them. They are all essential to soil ecology, breaking down toxic waste and recycling nutrients. They are even found in the atmosphere and one cubic metre of air holds around one hundred million bacterial cells. The oceans and seas harbour around 3 x 10 bacteria which provide up to 50% of the oxygen humans breathe. Only around 2% of bacterial species have been fully studied.


title: Bacteria

text: Size. Bacteria display a wide diversity of shapes and sizes. Bacterial cells are about one-tenth the size of eukaryotic cells and are typically 0.5–5.0 micrometres in length. However, a few species are visible to the unaided eye—for example, "Thiomargarita namibiensis" is up to half a millimetre long, "Epulopiscium fishelsoni" reaches 0.7 mm, and "Thiomargarita magnifica" can reach even 2 cm in length, which is 50 times larger than other known bacteria. Among the smallest bacteria are members of the genus "Mycoplasma", which measure only 0.3 micrometres, as small as the largest viruses. Some bacteria may be even smaller, but these ultramicrobacteria are not well-studied.


title: Bacteria

text: Shape. Most bacterial species are either spherical, called "cocci" ("singular coccus", from Greek "kókkos", grain, seed), or rod shaped, called "bacilli" ("sing". bacillus, from Latin "baculus", stick). Some bacteria, called "vibrio", are shaped like slightly curved rods or comma shaped; others can be spiral shaped, called "spirilla", or tightly coiled, called "spirochaetes". A small number of other unusual shapes have been described, such as star-shaped bacteria. This wide variety of shapes is determined by the bacterial cell wall and cytoskeleton, and is important because it can influence the ability of bacteria to acquire nutrients, attach to surfaces, swim through liquids and escape predators.


title: Bacteria

text: Multicellularity. Most bacterial species exist as single cells; others associate in characteristic patterns: "Neisseria" forms diploids (pairs), streptococci form chains, and staphylococci group together in "bunch of grapes" clusters. Bacteria can also group to form larger multicellular structures, such as the elongated filaments of "Actinomycetota" species, the aggregates of "Myxobacteria" species, and the complex hyphae of "Streptomyces" species. These multicellular structures are often only seen in certain conditions. For example, when starved of amino acids, myxobacteria detect surrounding cells in a process known as quorum sensing, migrate towards each other, and aggregate to form fruiting bodies up to 500 micrometres long and containing approximately 100,000 bacterial cells. In these fruiting bodies, the bacteria perform separate tasks; for example, about one in ten cells migrate to the top of a fruiting body and differentiate into a specialised dormant state called a myxospore, which is more resistant to drying and other adverse environmental conditions.


title: Bacteria

text: Biofilms. Bacteria often attach to surfaces and form dense aggregations called biofilms, and larger formations known as microbial mats. These biofilms and mats can range from a few micrometres in thickness to up to half a metre in depth, and may contain multiple species of bacteria, protists and archaea. Bacteria living in biofilms display a complex arrangement of cells and extracellular components, forming secondary structures, such as microcolonies, through which there are networks of channels to enable better diffusion of nutrients. In natural environments, such as soil or the surfaces of plants, the majority of bacteria are bound to surfaces in biofilms. Biofilms are also important in medicine, as these structures are often present during chronic bacterial infections or in infections of implanted medical devices, and bacteria protected within biofilms are much harder to kill than individual isolated bacteria.


title: Bacteria

text: The bacterial cell is surrounded by a cell membrane, which is made primarily of phospholipids. This membrane encloses the contents of the cell and acts as a barrier to hold nutrients, proteins and other essential components of the cytoplasm within the cell. Unlike eukaryotic cells, bacteria usually lack large membrane-bound structures in their cytoplasm such as a nucleus, mitochondria, chloroplasts and the other organelles present in eukaryotic cells. However, some bacteria have protein-bound organelles in the cytoplasm which compartmentalize aspects of bacterial metabolism, such as the carboxysome. Additionally, bacteria have a multi-component cytoskeleton to control the localisation of proteins and nucleic acids within the cell, and to manage the process of cell division.


title: Bacteria

text: Many important biochemical reactions, such as energy generation, occur due to concentration gradients across membranes, creating a potential difference analogous to a battery. The general lack of internal membranes in bacteria means these reactions, such as electron transport, occur across the cell membrane between the cytoplasm and the outside of the cell or periplasm. However, in many photosynthetic bacteria the plasma membrane is highly folded and fills most of the cell with layers of light-gathering membrane. These light-gathering complexes may even form lipid-enclosed structures called chlorosomes in green sulfur bacteria.


title: Bacteria

text: Bacteria do not have a membrane-bound nucleus, and their genetic material is typically a single circular bacterial chromosome of DNA located in the cytoplasm in an irregularly shaped body called the nucleoid. The nucleoid contains the chromosome with its associated proteins and RNA. Like all other organisms, bacteria contain ribosomes for the production of proteins, but the structure of the bacterial ribosome is different from that of eukaryotes and archaea.


title: Bacteria

text: Some bacteria produce intracellular nutrient storage granules, such as glycogen, polyphosphate, sulfur or polyhydroxyalkanoates. Bacteria such as the photosynthetic cyanobacteria, produce internal gas vacuoles, which they use to regulate their buoyancy, allowing them to move up or down into water layers with different light intensities and nutrient levels.


title: Bacteria

text: Around the outside of the cell membrane is the cell wall. Bacterial cell walls are made of peptidoglycan (also called murein), which is made from polysaccharide chains cross-linked by peptides containing D-amino acids. Bacterial cell walls are different from the cell walls of plants and fungi, which are made of cellulose and chitin, respectively. The cell wall of bacteria is also distinct from that of achaea, which do not contain peptidoglycan. The cell wall is essential to the survival of many bacteria, and the antibiotic penicillin (produced by a fungus called "Penicillium") is able to kill bacteria by inhibiting a step in the synthesis of peptidoglycan.


title: Bacteria

text: There are broadly speaking two different types of cell wall in bacteria, that classify bacteria into Gram-positive bacteria and Gram-negative bacteria. The names originate from the reaction of cells to the Gram stain, a long-standing test for the classification of bacterial species.


title: Bacteria

text: Gram-positive bacteria possess a thick cell wall containing many layers of peptidoglycan and teichoic acids. In contrast, Gram-negative bacteria have a relatively thin cell wall consisting of a few layers of peptidoglycan surrounded by a second lipid membrane containing lipopolysaccharides and lipoproteins. Most bacteria have the Gram-negative cell wall, and only members of the "Bacillota" group and actinomycetota (previously known as the low G+C and high G+C Gram-positive bacteria, respectively) have the alternative Gram-positive arrangement. These differences in structure can produce differences in antibiotic susceptibility; for instance, vancomycin can kill only Gram-positive bacteria and is ineffective against Gram-negative pathogens, such as "Haemophilus influenzae" or "Pseudomonas aeruginosa". Some bacteria have cell wall structures that are neither classically Gram-positive or Gram-negative. This includes clinically important bacteria such as mycobacteria which have a thick peptidoglycan cell wall like a Gram-positive bacterium, but also a second outer layer of lipids.


title: Bacteria

text: In many bacteria, an S-layer of rigidly arrayed protein molecules covers the outside of the cell. This layer provides chemical and physical protection for the cell surface and can act as a macromolecular diffusion barrier. S-layers have diverse functions and are known to act as virulence factors in "Campylobacter" species and contain surface enzymes in "Bacillus stearothermophilus".


title: Bacteria

text: Flagella are rigid protein structures, about 20 nanometres in diameter and up to 20 micrometres in length, that are used for motility. Flagella are driven by the energy released by the transfer of ions down an electrochemical gradient across the cell membrane.


title: Bacteria

text: Fimbriae (sometimes called "attachment pili") are fine filaments of protein, usually 2–10 nanometres in diameter and up to several micrometres in length. They are distributed over the surface of the cell, and resemble fine hairs when seen under the electron microscope. Fimbriae are believed to be involved in attachment to solid surfaces or to other cells, and are essential for the virulence of some bacterial pathogens. Pili ("sing". pilus) are cellular appendages, slightly larger than fimbriae, that can transfer genetic material between bacterial cells in a process called conjugation where they are called conjugation pili or sex pili (see bacterial genetics, below). They can also generate movement where they are called type IV pili.


title: Bacteria

text: Glycocalyx is produced by many bacteria to surround their cells, and varies in structural complexity: ranging from a disorganised slime layer of extracellular polymeric substances to a highly structured capsule. These structures can protect cells from engulfment by eukaryotic cells such as macrophages (part of the human immune system). They can also act as antigens and be involved in cell recognition, as well as aiding attachment to surfaces and the formation of biofilms.


title: Bacteria

text: The assembly of these extracellular structures is dependent on bacterial secretion systems. These transfer proteins from the cytoplasm into the periplasm or into the environment around the cell. Many types of secretion systems are known and these structures are often essential for the virulence of pathogens, so are intensively studied.


title: Bacteria

text: Some genera of Gram-positive bacteria, such as "Bacillus", "Clostridium", "Sporohalobacter", "Anaerobacter", and "Heliobacterium", can form highly resistant, dormant structures called "endospores". Endospores develop within the cytoplasm of the cell; generally a single endospore develops in each cell. Each endospore contains a core of DNA and ribosomes surrounded by a cortex layer and protected by a multilayer rigid coat composed of peptidoglycan and a variety of proteins.


title: Bacteria

text: Endospores show no detectable metabolism and can survive extreme physical and chemical stresses, such as high levels of UV light, gamma radiation, detergents, disinfectants, heat, freezing, pressure, and desiccation. In this dormant state, these organisms may remain viable for millions of years, and endospores even allow bacteria to survive exposure to the vacuum and radiation in space, possibly bacteria could be distributed throughout the Universe by space dust, meteoroids, asteroids, comets, planetoids or via directed panspermia. Endospore-forming bacteria can also cause disease: for example, anthrax can be contracted by the inhalation of "Bacillus anthracis" endospores, and contamination of deep puncture wounds with "Clostridium tetani" endospores causes tetanus, which like botulism is caused by a toxin released by the bacteria that grow from the spores. Clostridioides difficile infection, which is a problem in healthcare settings is also caused by spore-forming bacteria.


title: Bacteria

text: Bacteria exhibit an extremely wide variety of metabolic types. The distribution of metabolic traits within a group of bacteria has traditionally been used to define their taxonomy, but these traits often do not correspond with modern genetic classifications. Bacterial metabolism is classified into nutritional groups on the basis of three major criteria: the source of energy, the electron donors used, and the source of carbon used for growth.


title: Bacteria

text: Bacteria either derive energy from light using photosynthesis (called phototrophy), or by breaking down chemical compounds using oxidation (called chemotrophy). Chemotrophs use chemical compounds as a source of energy by transferring electrons from a given electron donor to a terminal electron acceptor in a redox reaction. This reaction releases energy that can be used to drive metabolism. Chemotrophs are further divided by the types of compounds they use to transfer electrons. Bacteria that use inorganic compounds such as hydrogen, carbon monoxide, or ammonia as sources of electrons are called lithotrophs, while those that use organic compounds are called organotrophs. The compounds used to receive electrons are also used to classify bacteria: aerobic organisms use oxygen as the terminal electron acceptor, while anaerobic organisms use other compounds such as nitrate, sulfate, or carbon dioxide.


title: Bacteria

text: Many bacteria get their carbon from other organic carbon, called heterotrophy. Others such as cyanobacteria and some purple bacteria are autotrophic, meaning that they obtain cellular carbon by fixing carbon dioxide. In unusual circumstances, the gas methane can be used by methanotrophic bacteria as both a source of electrons and a substrate for carbon anabolism.


title: Bacteria

text: In many ways, bacterial metabolism provides traits that are useful for ecological stability and for human society. One example is that some bacteria called diazotrophs have the ability to fix nitrogen gas using the enzyme nitrogenase. This environmentally important trait can be found in bacteria of most metabolic types listed above. This leads to the ecologically important processes of denitrification, sulfate reduction, and acetogenesis, respectively. Bacterial metabolic processes are also important in biological responses to pollution; for example, sulfate-reducing bacteria are largely responsible for the production of the highly toxic forms of mercury (methyl- and dimethylmercury) in the environment. Non-respiratory anaerobes use fermentation to generate energy and reducing power, secreting metabolic by-products (such as ethanol in brewing) as waste. Facultative anaerobes can switch between fermentation and different terminal electron acceptors depending on the environmental conditions in which they find themselves.


title: Bacteria

text: Unlike in multicellular organisms, increases in cell size (cell growth) and reproduction by cell division are tightly linked in unicellular organisms. Bacteria grow to a fixed size and then reproduce through binary fission, a form of asexual reproduction. Under optimal conditions, bacteria can grow and divide extremely rapidly, and some bacterial populations can double as quickly as every 17 minutes. In cell division, two identical clone daughter cells are produced. Some bacteria, while still reproducing asexually, form more complex reproductive structures that help disperse the newly formed daughter cells. Examples include fruiting body formation by myxobacteria and aerial hyphae formation by "Streptomyces" species, or budding. Budding involves a cell forming a protrusion that breaks away and produces a daughter cell.


title: Bacteria

text: In the laboratory, bacteria are usually grown using solid or liquid media. Solid growth media, such as agar plates, are used to isolate pure cultures of a bacterial strain. However, liquid growth media are used when the measurement of growth or large volumes of cells are required. Growth in stirred liquid media occurs as an even cell suspension, making the cultures easy to divide and transfer, although isolating single bacteria from liquid media is difficult. The use of selective media (media with specific nutrients added or deficient, or with antibiotics added) can help identify specific organisms.


title: Bacteria

text: Most laboratory techniques for growing bacteria use high levels of nutrients to produce large amounts of cells cheaply and quickly. However, in natural environments, nutrients are limited, meaning that bacteria cannot continue to reproduce indefinitely. This nutrient limitation has led the evolution of different growth strategies (see r/K selection theory). Some organisms can grow extremely rapidly when nutrients become available, such as the formation of algal and cyanobacterial blooms that often occur in lakes during the summer. Other organisms have adaptations to harsh environments, such as the production of multiple antibiotics by streptomyces that inhibit the growth of competing microorganisms. In nature, many organisms live in communities (e.g., biofilms) that may allow for increased supply of nutrients and protection from environmental stresses. These relationships can be essential for growth of a particular organism or group of organisms (syntrophy).


title: Bacteria

text: Bacterial growth follows four phases. When a population of bacteria first enter a high-nutrient environment that allows growth, the cells need to adapt to their new environment. The first phase of growth is the lag phase, a period of slow growth when the cells are adapting to the high-nutrient environment and preparing for fast growth. The lag phase has high biosynthesis rates, as proteins necessary for rapid growth are produced. The second phase of growth is the logarithmic phase, also known as the exponential phase. The log phase is marked by rapid exponential growth. The rate at which cells grow during this phase is known as the "growth rate" ("k"), and the time it takes the cells to double is known as the "generation time" ("g"). During log phase, nutrients are metabolised at maximum speed until one of the nutrients is depleted and starts limiting growth. The third phase of growth is the "stationary phase" and is caused by depleted nutrients. The cells reduce their metabolic activity and consume non-essential cellular proteins. The stationary phase is a transition from rapid growth to a stress response state and there is increased expression of genes involved in DNA repair, antioxidant metabolism and nutrient transport. The final phase is the death phase where the bacteria run out of nutrients and die.


title: Bacteria

text: Most bacteria have a single circular chromosome that can range in size from only 160,000 base pairs in the endosymbiotic bacteria "Carsonella ruddii", to 12,200,000 base pairs (12.2 Mbp) in the soil-dwelling bacteria "Sorangium cellulosum". There are many exceptions to this, for example some "Streptomyces" and "Borrelia" species contain a single linear chromosome, while some "Vibrio" species contain more than one chromosome. Bacteria can also contain plasmids, small extra-chromosomal molecules of DNA that may contain genes for various useful functions such as antibiotic resistance, metabolic capabilities, or various virulence factors.


title: Bacteria

text: Bacteria genomes usually encode a few hundred to a few thousand genes. The genes in bacterial genomes are usually a single continuous stretch of DNA and although several different types of introns do exist in bacteria, these are much rarer than in eukaryotes.


title: Bacteria

text: Bacteria, as asexual organisms, inherit an identical copy of the parent's genomes and are clonal. However, all bacteria can evolve by selection on changes to their genetic material DNA caused by genetic recombination or mutations. Mutations come from errors made during the replication of DNA or from exposure to mutagens. Mutation rates vary widely among different species of bacteria and even among different clones of a single species of bacteria. Genetic changes in bacterial genomes come from either random mutation during replication or "stress-directed mutation", where genes involved in a particular growth-limiting process have an increased mutation rate.


title: Bacteria

text: Some bacteria also transfer genetic material between cells. This can occur in three main ways. First, bacteria can take up exogenous DNA from their environment, in a process called transformation. Many bacteria can naturally take up DNA from the environment, while others must be chemically altered in order to induce them to take up DNA. The development of competence in nature is usually associated with stressful environmental conditions, and seems to be an adaptation for facilitating repair of DNA damage in recipient cells. The second way bacteria transfer genetic material is by transduction, when the integration of a bacteriophage introduces foreign DNA into the chromosome. Many types of bacteriophage exist, some infect and lyse their host bacteria, while others insert into the bacterial chromosome. Bacteria resist phage infection through restriction modification systems that degrade foreign DNA, and a system that uses CRISPR sequences to retain fragments of the genomes of phage that the bacteria have come into contact with in the past, which allows them to block virus replication through a form of RNA interference. The third method of gene transfer is conjugation, whereby DNA is transferred through direct cell contact. In ordinary circumstances, transduction, conjugation, and transformation involve transfer of DNA between individual bacteria of the same species, but occasionally transfer may occur between individuals of different bacterial species and this may have significant consequences, such as the transfer of antibiotic resistance. In such cases, gene acquisition from other bacteria or the environment is called horizontal gene transfer and may be common under natural conditions.


title: Bacteria

text: Many bacteria are motile (able to move themselves) and do so using a variety of mechanisms. The best studied of these are flagella, long filaments that are turned by a motor at the base to generate propeller-like movement. The bacterial flagellum is made of about 20 proteins, with approximately another 30 proteins required for its regulation and assembly. The flagellum is a rotating structure driven by a reversible motor at the base that uses the electrochemical gradient across the membrane for power.


title: Bacteria

text: Bacteria can use flagella in different ways to generate different kinds of movement. Many bacteria (such as "E. coli") have two distinct modes of movement: forward movement (swimming) and tumbling. The tumbling allows them to reorient and makes their movement a three-dimensional random walk. Bacterial species differ in the number and arrangement of flagella on their surface; some have a single flagellum ("monotrichous"), a flagellum at each end ("amphitrichous"), clusters of flagella at the poles of the cell ("lophotrichous"), while others have flagella distributed over the entire surface of the cell ("peritrichous"). The flagella of a unique group of bacteria, the spirochaetes, are found between two membranes in the periplasmic space. They have a distinctive helical body that twists about as it moves.


title: Bacteria

text: Two other types of bacterial motion are called twitching motility that relies on a structure called the type IV pilus, and gliding motility, that uses other mechanisms. In twitching motility, the rod-like pilus extends out from the cell, binds some substrate, and then retracts, pulling the cell forward.


title: Bacteria

text: Motile bacteria are attracted or repelled by certain stimuli in behaviours called "taxes": these include chemotaxis, phototaxis, energy taxis, and magnetotaxis. In one peculiar group, the myxobacteria, individual bacteria move together to form waves of cells that then differentiate to form fruiting bodies containing spores. The myxobacteria move only when on solid surfaces, unlike "E. coli", which is motile in liquid or solid media.


title: Bacteria

text: Several "Listeria" and "Shigella" species move inside host cells by usurping the cytoskeleton, which is normally used to move organelles inside the cell. By promoting actin polymerisation at one pole of their cells, they can form a kind of tail that pushes them through the host cell's cytoplasm.


title: Bacteria

text: A few bacteria have chemical systems that generate light. This bioluminescence often occurs in bacteria that live in association with fish, and the light probably serves to attract fish or other large animals.


title: Bacteria

text: Bacteria often function as multicellular aggregates known as biofilms, exchanging a variety of molecular signals for inter-cell communication, and engaging in coordinated multicellular behaviour.


title: Bacteria

text: The communal benefits of multicellular cooperation include a cellular division of labour, accessing resources that cannot effectively be used by single cells, collectively defending against antagonists, and optimising population survival by differentiating into distinct cell types. For example, bacteria in biofilms can have more than 500 times increased resistance to antibacterial agents than individual "planktonic" bacteria of the same species.


title: Bacteria

text: One type of inter-cellular communication by a molecular signal is called quorum sensing, which serves the purpose of determining whether there is a local population density that is sufficiently high that it is productive to invest in processes that are only successful if large numbers of similar organisms behave similarly, as in excreting digestive enzymes or emitting light.


title: Bacteria

text: Quorum sensing allows bacteria to coordinate gene expression, and enables them to produce, release and detect autoinducers or pheromones which accumulate with the growth in cell population.


title: Bacteria

text: Classification seeks to describe the diversity of bacterial species by naming and grouping organisms based on similarities. Bacteria can be classified on the basis of cell structure, cellular metabolism or on differences in cell components, such as DNA, fatty acids, pigments, antigens and quinones. While these schemes allowed the identification and classification of bacterial strains, it was unclear whether these differences represented variation between distinct species or between strains of the same species. This uncertainty was due to the lack of distinctive structures in most bacteria, as well as lateral gene transfer between unrelated species. Due to lateral gene transfer, some closely related bacteria can have very different morphologies and metabolisms. To overcome this uncertainty, modern bacterial classification emphasises molecular systematics, using genetic techniques such as guanine cytosine ratio determination, genome-genome hybridisation, as well as sequencing genes that have not undergone extensive lateral gene transfer, such as the rRNA gene. Classification of bacteria is determined by publication in the International Journal of Systematic Bacteriology, and Bergey's Manual of Systematic Bacteriology. The International Committee on Systematic Bacteriology (ICSB) maintains international rules for the naming of bacteria and taxonomic categories and for the ranking of them in the International Code of Nomenclature of Bacteria.


title: Bacteria

text: Historically, bacteria were considered a part of the Plantae, the Plant kingdom, and were called "Schizomycetes" (fission-fungi). For this reason, collective bacteria and other microorganisms in a host are often called "flora".


title: Bacteria

text: The term "bacteria" was traditionally applied to all microscopic, single-cell prokaryotes. However, molecular systematics showed prokaryotic life to consist of two separate domains, originally called Eubacteria and Archaebacteria, but now called Bacteria and Archaea that evolved independently from an ancient common ancestor. The archaea and eukaryotes are more closely related to each other than either is to the bacteria. These two domains, along with Eukarya, are the basis of the three-domain system, which is currently the most widely used classification system in microbiology. However, due to the relatively recent introduction of molecular systematics and a rapid increase in the number of genome sequences that are available, bacterial classification remains a changing and expanding field. For example, Cavalier-Smith argued that the Archaea and Eukaryotes evolved from Gram-positive bacteria.


title: Bacteria

text: The identification of bacteria in the laboratory is particularly relevant in medicine, where the correct treatment is determined by the bacterial species causing an infection. Consequently, the need to identify human pathogens was a major impetus for the development of techniques to identify bacteria.


title: Bacteria

text: The "Gram stain", developed in 1884 by Hans Christian Gram, characterises bacteria based on the structural characteristics of their cell walls. The thick layers of peptidoglycan in the "Gram-positive" cell wall stain purple, while the thin "Gram-negative" cell wall appears pink. By combining morphology and Gram-staining, most bacteria can be classified as belonging to one of four groups (Gram-positive cocci, Gram-positive bacilli, Gram-negative cocci and Gram-negative bacilli). Some organisms are best identified by stains other than the Gram stain, particularly mycobacteria or "Nocardia", which show acid fastness on Ziehl–Neelsen or similar stains. Other organisms may need to be identified by their growth in special media, or by other techniques, such as serology.


title: Bacteria

text: Culture techniques are designed to promote the growth and identify particular bacteria, while restricting the growth of the other bacteria in the sample. Often these techniques are designed for specific specimens; for example, a sputum sample will be treated to identify organisms that cause pneumonia, while stool specimens are cultured on selective media to identify organisms that cause diarrhea, while preventing growth of non-pathogenic bacteria. Specimens that are normally sterile, such as blood, urine or spinal fluid, are cultured under conditions designed to grow all possible organisms. Once a pathogenic organism has been isolated, it can be further characterised by its morphology, growth patterns (such as aerobic or anaerobic growth), patterns of hemolysis, and staining.


title: Bacteria

text: As with bacterial classification, identification of bacteria is increasingly using molecular methods, and mass spectroscopy. Most bacteria have not been characterised and there are may species that cannot be grown in the laboratory. Diagnostics using DNA-based tools, such as polymerase chain reaction, are increasingly popular due to their specificity and speed, compared to culture-based methods. These methods also allow the detection and identification of "viable but nonculturable" cells that are metabolically active but non-dividing. However, even using these improved methods, the total number of bacterial species is not known and cannot even be estimated with any certainty. Following present classification, there are a little less than 9,300 known species of prokaryotes, which includes bacteria and archaea; but attempts to estimate the true number of bacterial diversity have ranged from 10 to 10 total species—and even these diverse estimates may be off by many orders of magnitude.


title: Bacteria

text: The following phyla have been proposed, but have not been validly published according to the Bacteriological Code (including those that have "candidatus" status):


title: Bacteria

text: Despite their apparent simplicity, bacteria can form complex associations with other organisms. These symbiotic associations can be divided into parasitism, mutualism and commensalism.


title: Bacteria

text: The word "commensalism" is derived from the word "commensal", meaning "eating at the same table" and all plants and animals are colonised by commensal bacteria. In humans and other animals millions of them live on the skin, the airways, the gut and other orifices.


title: Bacteria

text: Referred to as "normal flora", or "commensals", these bacteria usually cause no harm but may occasionally invade other sites of the body and cause infection. "Escherichia coli" is a commensal in the human gut but can cause urinary tract infections. Similarly, streptoccoci, which are part of the normal flora of the human mouth, can cause heart disease.


title: Bacteria

text: Some species of bacteria kill and then consume other microorganisms, these species are called "predatory bacteria". These include organisms such as "Myxococcus xanthus", which forms swarms of cells that kill and digest any bacteria they encounter. Other bacterial predators either attach to their prey in order to digest them and absorb nutrients or invade another cell and multiply inside the cytosol. These predatory bacteria are thought to have evolved from saprophages that consumed dead microorganisms, through adaptations that allowed them to entrap and kill other organisms.


title: Bacteria

text: Certain bacteria form close spatial associations that are essential for their survival. One such mutualistic association, called interspecies hydrogen transfer, occurs between clusters of anaerobic bacteria that consume organic acids, such as butyric acid or propionic acid, and produce hydrogen, and methanogenic archaea that consume hydrogen. The bacteria in this association are unable to consume the organic acids as this reaction produces hydrogen that accumulates in their surroundings. Only the intimate association with the hydrogen-consuming archaea keeps the hydrogen concentration low enough to allow the bacteria to grow.


title: Bacteria

text: In soil, microorganisms that reside in the rhizosphere (a zone that includes the root surface and the soil that adheres to the root after gentle shaking) carry out nitrogen fixation, converting nitrogen gas to nitrogenous compounds. This serves to provide an easily absorbable form of nitrogen for many plants, which cannot fix nitrogen themselves. Many other bacteria are found as symbionts in humans and other organisms. For example, the presence of over 1,000 bacterial species in the normal human gut flora of the intestines can contribute to gut immunity, synthesise vitamins, such as folic acid, vitamin K and biotin, convert sugars to lactic acid (see "Lactobacillus"), as well as fermenting complex undigestible carbohydrates. The presence of this gut flora also inhibits the growth of potentially pathogenic bacteria (usually through competitive exclusion) and these beneficial bacteria are consequently sold as probiotic dietary supplements.


title: Bacteria

text: Nearly all animal life is dependent on bacteria for survival as only bacteria and some archaea possess the genes and enzymes necessary to synthesize vitamin B, also known as cobalamin, and provide it through the food chain. Vitamin B is a water-soluble vitamin that is involved in the metabolism of every cell of the human body. It is a cofactor in DNA synthesis, and in both fatty acid and amino acid metabolism. It is particularly important in the normal functioning of the nervous system via its role in the synthesis of myelin.


title: Bacteria

text: The body is continually exposed to many species of bacteria, including beneficial commensals, which grow on the skin and mucous membranes, and saprophytes, which grow mainly in the soil and in decaying matter. The blood and tissue fluids contain nutrients sufficient to sustain the growth of many bacteria. The body has defence mechanisms that enable it to resist microbial invasion of its tissues and give it a natural immunity or innate resistance against many microorganisms. Unlike some viruses, bacteria evolve relatively slowly so many bacterial diseases also occur in other animals.


title: Bacteria

text: If bacteria form a parasitic association with other organisms, they are classed as pathogens. Pathogenic bacteria are a major cause of human death and disease and cause infections such as tetanus (caused by "Clostridium tetani"), typhoid fever, diphtheria, syphilis, cholera, foodborne illness, leprosy (caused by "Mycobacterium leprae") and tuberculosis (caused by "Mycobacterium tuberculosis"). A pathogenic cause for a known medical disease may only be discovered many years later, as was the case with "Helicobacter pylori" and peptic ulcer disease. Bacterial diseases are also important in agriculture, with bacteria causing leaf spot, fire blight and wilts in plants, as well as Johne's disease, mastitis, salmonella and anthrax in farm animals.


title: Bacteria

text: Each species of pathogen has a characteristic spectrum of interactions with its human hosts. Some organisms, such as "Staphylococcus" or "Streptococcus", can cause skin infections, pneumonia, meningitis and sepsis, a systemic inflammatory response producing shock, massive vasodilation and death. Yet these organisms are also part of the normal human flora and usually exist on the skin or in the nose without causing any disease at all. Other organisms invariably cause disease in humans, such as "Rickettsia", which are obligate intracellular parasites able to grow and reproduce only within the cells of other organisms. One species of "Rickettsia" causes typhus, while another causes Rocky Mountain spotted fever. "Chlamydia", another phylum of obligate intracellular parasites, contains species that can cause pneumonia or urinary tract infection and may be involved in coronary heart disease. Some species, such as "Pseudomonas aeruginosa", "Burkholderia cenocepacia", and "Mycobacterium avium", are opportunistic pathogens and cause disease mainly in people who are immunosuppressed or have cystic fibrosis. Some bacteria produce toxins, which cause diseases. These are endotoxins, which come from broken bacterial cells, and exotoxins, which are produced by bacteria and released into the environment. The bacterium "Clostridium botulinum" for example, produces a powerful exotoxin that cause respiratory paralysis, and "Salmonellae" produce an endotoxin that causes gastroenteritis. Some exotoxins can be converted to toxoids, which are used as vaccines to prevent the disease.


title: Bacteria

text: Bacterial infections may be treated with antibiotics, which are classified as bacteriocidal if they kill bacteria or bacteriostatic if they just prevent bacterial growth. There are many types of antibiotics, and each class inhibits a process that is different in the pathogen from that found in the host. An example of how antibiotics produce selective toxicity are chloramphenicol and puromycin, which inhibit the bacterial ribosome, but not the structurally different eukaryotic ribosome. Antibiotics are used both in treating human disease and in intensive farming to promote animal growth, where they may be contributing to the rapid development of antibiotic resistance in bacterial populations. Infections can be prevented by antiseptic measures such as sterilising the skin prior to piercing it with the needle of a syringe, and by proper care of indwelling catheters. Surgical and dental instruments are also sterilised to prevent contamination by bacteria. Disinfectants such as bleach are used to kill bacteria or other pathogens on surfaces to prevent contamination and further reduce the risk of infection.


title: Bacteria

text: Bacteria, often lactic acid bacteria, such as "Lactobacillus" species and "Lactococcus" species, in combination with yeasts and moulds, have been used for thousands of years in the preparation of fermented foods, such as cheese, pickles, soy sauce, sauerkraut, vinegar, wine and yogurt.


title: Bacteria

text: The ability of bacteria to degrade a variety of organic compounds is remarkable and has been used in waste processing and bioremediation. Bacteria capable of digesting the hydrocarbons in petroleum are often used to clean up oil spills. Fertiliser was added to some of the beaches in Prince William Sound in an attempt to promote the growth of these naturally occurring bacteria after the 1989 "Exxon Valdez" oil spill. These efforts were effective on beaches that were not too thickly covered in oil. Bacteria are also used for the bioremediation of industrial toxic wastes. In the chemical industry, bacteria are most important in the production of enantiomerically pure chemicals for use as pharmaceuticals or agrichemicals.


title: Bacteria

text: Bacteria can also be used in the place of pesticides in the biological pest control. This commonly involves "Bacillus thuringiensis" (also called BT), a Gram-positive, soil dwelling bacterium. Subspecies of this bacteria are used as a Lepidopteran-specific insecticides under trade names such as Dipel and Thuricide. Because of their specificity, these pesticides are regarded as environmentally friendly, with little or no effect on humans, wildlife, pollinators and most other beneficial insects.


title: Bacteria

text: Because of their ability to quickly grow and the relative ease with which they can be manipulated, bacteria are the workhorses for the fields of molecular biology, genetics and biochemistry. By making mutations in bacterial DNA and examining the resulting phenotypes, scientists can determine the function of genes, enzymes and metabolic pathways in bacteria, then apply this knowledge to more complex organisms. This aim of understanding the biochemistry of a cell reaches its most complex expression in the synthesis of huge amounts of enzyme kinetic and gene expression data into mathematical models of entire organisms. This is achievable in some well-studied bacteria, with models of "Escherichia coli" metabolism now being produced and tested. This understanding of bacterial metabolism and genetics allows the use of biotechnology to bioengineer bacteria for the production of therapeutic proteins, such as insulin, growth factors, or antibodies.


title: Bacteria

text: Because of their importance for research in general, samples of bacterial strains are isolated and preserved in Biological Resource Centers. This ensures the availability of the strain to scientists worldwide.


title: Bacteria

text: Bacteria were first observed by the Dutch microscopist Antonie van Leeuwenhoek in 1676, using a single-lens microscope of his own design. He then published his observations in a series of letters to the Royal Society of London. Bacteria were Leeuwenhoek's most remarkable microscopic discovery. They were just at the limit of what his simple lenses could make out and, in one of the most striking hiatuses in the history of science, no one else would see them again for over a century. His observations had also included protozoans which he called animalcules, and his findings were looked at again in the light of the more recent findings of cell theory.


title: Bacteria

text: Christian Gottfried Ehrenberg introduced the word "bacterium" in 1828. In fact, his "Bacterium" was a genus that contained non-spore-forming rod-shaped bacteria, as opposed to "Bacillus", a genus of spore-forming rod-shaped bacteria defined by Ehrenberg in 1835.


title: Bacteria

text: Louis Pasteur demonstrated in 1859 that the growth of microorganisms causes the fermentation process, and that this growth is not due to spontaneous generation (yeasts and molds, commonly associated with fermentation, are not bacteria, but rather fungi). Along with his contemporary Robert Koch, Pasteur was an early advocate of the germ theory of disease. Before them, Ignaz Semmelweis and Joseph Lister had realised the importance of sanitized hands in medical work. Semmelweis ideas was rejected and his book on the topic condemned by the medical community, but after Lister doctors started sanitizing their hands in the 1870s. While Semmelweis who started with rules about handwashing in his hospital in the 1840s predated the spread of the ideas about germs themselves and attributed diseases to "decomposing animal organic matter", Lister was active later.


title: Bacteria

text: Robert Koch, a pioneer in medical microbiology, worked on cholera, anthrax and tuberculosis. In his research into tuberculosis Koch finally proved the germ theory, for which he received a Nobel Prize in 1905. In Koch's postulates, he set out criteria to test if an organism is the cause of a disease, and these postulates are still used today.


title: Bacteria

text: Ferdinand Cohn is said to be a founder of bacteriology, studying bacteria from 1870. Cohn was the first to classify bacteria based on their morphology.


title: Bacteria

text: Though it was known in the nineteenth century that bacteria are the cause of many diseases, no effective antibacterial treatments were available. In 1910, Paul Ehrlich developed the first antibiotic, by changing dyes that selectively stained "Treponema pallidum"—the spirochaete that causes syphilis—into compounds that selectively killed the pathogen. Ehrlich had been awarded a 1908 Nobel Prize for his work on immunology, and pioneered the use of stains to detect and identify bacteria, with his work being the basis of the Gram stain and the Ziehl–Neelsen stain.


title: Bacteria

text: A major step forward in the study of bacteria came in 1977 when Carl Woese recognised that archaea have a separate line of evolutionary descent from bacteria. This new phylogenetic taxonomy depended on the sequencing of 16S ribosomal RNA, and divided prokaryotes into two evolutionary domains, as part of the three-domain system.


title: Electromagnetic spectrum

text: The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies.


title: Electromagnetic spectrum

text: The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from below one hertz to above 10 hertz, corresponding to wavelengths from thousands of kilometers down to a fraction of the size of an atomic nucleus. This frequency range is divided into separate bands, and the electromagnetic waves within each frequency band are called by different names; beginning at the low frequency (long wavelength) end of the spectrum these are: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays at the high-frequency (short wavelength) end. The electromagnetic waves in each of these bands have different characteristics, such as how they are produced, how they interact with matter, and their practical applications. There is no known limit for long and short wavelengths. Extreme ultraviolet, soft X-rays, hard X-rays and gamma rays are classified as ionizing radiation because their photons have enough energy to ionize atoms, causing chemical reactions. Exposure to ionizing radiation can be a health hazard, causing radiation sickness, DNA damage and cancer. Radiation of visible light and longer wavelengths are classified as nonionizing radiation because they have insufficient energy to cause these effects.


title: Electromagnetic spectrum

text: Throughout most of the electromagnetic spectrum, spectroscopy can be used to separate waves of different frequencies, producing a spectrum of the constituent frequencies. Spectroscopy is used to study the interactions of electromagnetic waves with matter.


title: Electromagnetic spectrum

text: Humans have always been aware of visible light and radiant heat but for most of history it was not known that these phenomena were connected or were representatives of a more extensive principle. The ancient Greeks recognized that light traveled in straight lines and studied some of its properties, including reflection and refraction. Light was intensively studied from the beginning of the 17th century leading to the invention of important instruments like the telescope and microscope. Isaac Newton was the first to use the term "spectrum" for the range of colours that white light could be split into with a prism. Starting in 1666, Newton showed that these colours were intrinsic to light and could be recombined into white light. A debate arose over whether light had a wave nature or a particle nature with René Descartes, Robert Hooke and Christiaan Huygens favouring a wave description and Newton favouring a particle description. Huygens in particular had a well developed theory from which he was able to derive the laws of reflection and refraction. Around 1801, Thomas Young measured the wavelength of a light beam with his two-slit experiment thus conclusively demonstrating that light was a wave.


title: Electromagnetic spectrum

text: In 1800, William Herschel discovered infrared radiation. He was studying the temperature of different colours by moving a thermometer through light split by a prism. He noticed that the highest temperature was beyond red. He theorized that this temperature change was due to "calorific rays", a type of light ray that could not be seen. The next year, Johann Ritter, working at the other end of the spectrum, noticed what he called "chemical rays" (invisible light rays that induced certain chemical reactions). These behaved similarly to visible violet light rays, but were beyond them in the spectrum. They were later renamed ultraviolet radiation.


title: Electromagnetic spectrum

text: The study of electromagnetism began in 1820 when Hans Christian Ørsted discovered that electric currents produce magnetic fields (Oersted's law). Light was first linked to electromagnetism in 1845, when Michael Faraday noticed that the polarization of light traveling through a transparent material responded to a magnetic field (see Faraday effect). During the 1860s, James Clerk Maxwell developed four partial differential equations (Maxwell's equations) for the electromagnetic field. Two of these equations predicted the possibility and behavior of waves in the field. Analyzing the speed of these theoretical waves, Maxwell realized that they must travel at a speed that was about the known speed of light. This startling coincidence in value led Maxwell to make the inference that light itself is a type of electromagnetic wave. Maxwell's equations predicted an infinite range of frequencies of electromagnetic waves, all traveling at the speed of light. This was the first indication of the existence of the entire electromagnetic spectrum.


title: Electromagnetic spectrum

text: Maxwell's predicted waves included waves at very low frequencies compared to infrared, which in theory might be created by oscillating charges in an ordinary electrical circuit of a certain type. Attempting to prove Maxwell's equations and detect such low frequency electromagnetic radiation, in 1886, the physicist Heinrich Hertz built an apparatus to generate and detect what are now called radio waves. Hertz found the waves and was able to infer (by measuring their wavelength and multiplying it by their frequency) that they traveled at the speed of light. Hertz also demonstrated that the new radiation could be both reflected and refracted by various dielectric media, in the same manner as light. For example, Hertz was able to focus the waves using a lens made of tree resin. In a later experiment, Hertz similarly produced and measured the properties of microwaves. These new types of waves paved the way for inventions such as the wireless telegraph and the radio.


title: Electromagnetic spectrum

text: In 1895, Wilhelm Röntgen noticed a new type of radiation emitted during an experiment with an evacuated tube subjected to a high voltage. He called this radiation "x-rays" and found that they were able to travel through parts of the human body but were reflected or stopped by denser matter such as bones. Before long, many uses were found for this radiography.


title: Electromagnetic spectrum

text: The last portion of the electromagnetic spectrum was filled in with the discovery of gamma rays. In 1900, Paul Villard was studying the radioactive emissions of radium when he identified a new type of radiation that he at first thought consisted of particles similar to known alpha and beta particles, but with the power of being far more penetrating than either. However, in 1910, British physicist William Henry Bragg demonstrated that gamma rays are electromagnetic radiation, not particles, and in 1914, Ernest Rutherford (who had named them gamma rays in 1903 when he realized that they were fundamentally different from charged alpha and beta particles) and Edward Andrade measured their wavelengths, and found that gamma rays were similar to X-rays, but with shorter wavelengths.


title: Electromagnetic spectrum

text: The wave-particle debate was rekindled in 1901 when Max Planck discovered that light is only absorbed in discrete "quanta", now called photons, implying that light has a particle nature. This idea was made explicit by Albert Einstein in 1905, but never accepted by Planck and many other contemporaries. The modern position of science is that electromagnetic radiation has both a wave and a particle nature, the wave-particle duality. The contradictions arising from this position are still being debated by scientists and philosophers.


title: Electromagnetic spectrum

text: Electromagnetic waves are typically described by any of the following three physical properties: the frequency "f", wavelength λ, or photon energy "E". Frequencies observed in astronomy range from (1 GeV gamma rays) down to the local plasma frequency of the ionized interstellar medium (~1 kHz). Wavelength is inversely proportional to the wave frequency, so gamma rays have very short wavelengths that are fractions of the size of atoms, whereas wavelengths on the opposite end of the spectrum can be indefinitely long. Photon energy is directly proportional to the wave frequency, so gamma ray photons have the highest energy (around a billion electron volts), while radio wave photons have very low energy (around a femtoelectronvolt). These relations are illustrated by the following equations:


title: Electromagnetic spectrum

text: Whenever electromagnetic waves exist in a medium with matter, their wavelength is decreased. Wavelengths of electromagnetic radiation, whatever medium they are traveling through, are usually quoted in terms of the "vacuum wavelength", although this is not always explicitly stated.


title: Electromagnetic spectrum

text: Generally, electromagnetic radiation is classified by wavelength into radio wave, microwave, infrared, visible light, ultraviolet, X-rays and gamma rays. The behavior of EM radiation depends on its wavelength. When EM radiation interacts with single atoms and molecules, its behavior also depends on the amount of energy per quantum (photon) it carries.


title: Electromagnetic spectrum

text: Spectroscopy can detect a much wider region of the EM spectrum than the visible wavelength range of 400 nm to 700 nm in a vacuum. A common laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm. Detailed information about the physical properties of objects, gases, or even stars can be obtained from this type of device. Spectroscopes are widely used in astrophysics. For example, many hydrogen atoms emit a radio wave photon that has a wavelength of 21.12 cm. Also, frequencies of 30 Hz and below can be produced by and are important in the study of certain stellar nebulae and frequencies as high as have been detected from astrophysical sources.


title: Electromagnetic spectrum

text: The types of electromagnetic radiation are broadly classified into the following classes (regions, bands or types):


title: Electromagnetic spectrum

text: This classification goes in the increasing order of wavelength, which is characteristic of the type of radiation.


title: Electromagnetic spectrum

text: There are no precisely defined boundaries between the bands of the electromagnetic spectrum; rather they fade into each other like the bands in a rainbow (which is the sub-spectrum of visible light). Radiation of each frequency and wavelength (or in each band) has a mix of properties of the two regions of the spectrum that bound it. For example, red light resembles infrared radiation in that it can excite and add energy to some chemical bonds and indeed must do so to power the chemical mechanisms responsible for photosynthesis and the working of the visual system.


title: Electromagnetic spectrum

text: The distinction between X-rays and gamma rays is partly based on sources: the photons generated from nuclear decay or other nuclear and subnuclear/particle process are always termed gamma rays, whereas X-rays are generated by electronic transitions involving highly energetic inner atomic electrons. In general, nuclear transitions are much more energetic than electronic transitions, so gamma rays are more energetic than X-rays, but exceptions exist. By analogy to electronic transitions, muonic atom transitions are also said to produce X-rays, even though their energy may exceed , whereas there are many (77 known to be less than ) low-energy nuclear transitions ("e.g.", the nuclear transition of thorium-229m), and, despite being one million-fold less energetic than some muonic X-rays, the emitted photons are still called gamma rays due to their nuclear origin.


title: Electromagnetic spectrum

text: The convention that EM radiation that is known to come from the nucleus is always called "gamma ray" radiation is the only convention that is universally respected, however. Many astronomical gamma ray sources (such as gamma ray bursts) are known to be too energetic (in both intensity and wavelength) to be of nuclear origin. Quite often, in high-energy physics and in medical radiotherapy, very high energy EMR (in the > 10 MeV region)—which is of higher energy than any nuclear gamma ray—is not called X-ray or gamma ray, but instead by the generic term of "high-energy photons".


title: Electromagnetic spectrum

text: The region of the spectrum where a particular observed electromagnetic radiation falls is reference frame-dependent (due to the Doppler shift for light), so EM radiation that one observer would say is in one region of the spectrum could appear to an observer moving at a substantial fraction of the speed of light with respect to the first to be in another part of the spectrum. For example, consider the cosmic microwave background. It was produced when matter and radiation decoupled, by the de-excitation of hydrogen atoms to the ground state. These photons were from Lyman series transitions, putting them in the ultraviolet (UV) part of the electromagnetic spectrum. Now this radiation has undergone enough cosmological red shift to put it into the microwave region of the spectrum for observers moving slowly (compared to the speed of light) with respect to the cosmos.


title: Electromagnetic spectrum

text: Electromagnetic radiation interacts with matter in different ways across the spectrum. These types of interaction are so different that historically different names have been applied to different parts of the spectrum, as though these were different types of radiation. Thus, although these "different kinds" of electromagnetic radiation form a quantitatively continuous spectrum of frequencies and wavelengths, the spectrum remains divided for practical reasons related to these qualitative interaction differences.


title: Electromagnetic spectrum

text: Radio waves are emitted and received by antennas, which consist of conductors such as metal rod resonators. In artificial generation of radio waves, an electronic device called a transmitter generates an AC electric current which is applied to an antenna. The oscillating electrons in the antenna generate oscillating electric and magnetic fields that radiate away from the antenna as radio waves. In reception of radio waves, the oscillating electric and magnetic fields of a radio wave couple to the electrons in an antenna, pushing them back and forth, creating oscillating currents which are applied to a radio receiver. Earth's atmosphere is mainly transparent to radio waves, except for layers of charged particles in the ionosphere which can reflect certain frequencies.


title: Electromagnetic spectrum

text: Radio waves are extremely widely used to transmit information across distances in radio communication systems such as radio broadcasting, television, two way radios, mobile phones, communication satellites, and wireless networking. In a radio communication system, a radio frequency current is modulated with an information-bearing signal in a transmitter by varying either the amplitude, frequency or phase, and applied to an antenna. The radio waves carry the information across space to a receiver, where they are received by an antenna and the information extracted by demodulation in the receiver. Radio waves are also used for navigation in systems like Global Positioning System (GPS) and navigational beacons, and locating distant objects in radiolocation and radar. They are also used for remote control, and for industrial heating.


title: Electromagnetic spectrum

text: The use of the radio spectrum is strictly regulated by governments, coordinated by the International Telecommunication Union (ITU) which allocates frequencies to different users for different uses.


title: Electromagnetic spectrum

text: Microwaves are radio waves of short wavelength, from about 10 centimeters to one millimeter, in the SHF and EHF frequency bands. Microwave energy is produced with klystron and magnetron tubes, and with solid state devices such as Gunn and IMPATT diodes. Although they are emitted and absorbed by short antennas, they are also absorbed by polar molecules, coupling to vibrational and rotational modes, resulting in bulk heating. Unlike higher frequency waves such as infrared and light which are absorbed mainly at surfaces, microwaves can penetrate into materials and deposit their energy below the surface. This effect is used to heat food in microwave ovens, and for industrial heating and medical diathermy. Microwaves are the main wavelengths used in radar, and are used for satellite communication, and wireless networking technologies such as Wi-Fi. The copper cables (transmission lines) which are used to carry lower frequency radio waves to antennas have excessive power losses at microwave frequencies, and metal pipes called waveguides are used to carry them. Although at the low end of the band the atmosphere is mainly transparent, at the upper end of the band absorption of microwaves by atmospheric gasses limits practical propagation distances to a few kilometers.


title: Electromagnetic spectrum

text: Terahertz radiation or sub-millimeter radiation is a region of the spectrum from about 100 GHz to 30 terahertz (THz) between microwaves and far infrared which can be regarded as belonging to either band. Until recently, the range was rarely studied and few sources existed for microwave energy in the so-called "terahertz gap", but applications such as imaging and communications are now appearing. Scientists are also looking to apply terahertz technology in the armed forces, where high-frequency waves might be directed at enemy troops to incapacitate their electronic equipment. Terahertz radiation is strongly absorbed by atmospheric gases, making this frequency range useless for long-distance communication.


title: Electromagnetic spectrum

text: The infrared part of the electromagnetic spectrum covers the range from roughly 300 GHz to 400 THz (1 mm – 750 nm). It can be divided into three parts:


title: Electromagnetic spectrum

text: Above infrared in frequency comes visible light. The Sun emits its peak power in the visible region, although integrating the entire emission power spectrum through all wavelengths shows that the Sun emits slightly more infrared than visible light. By definition, visible light is the part of the EM spectrum the human eye is the most sensitive to. Visible light (and near-infrared light) is typically absorbed and emitted by electrons in molecules and atoms that move from one energy level to another. This action allows the chemical mechanisms that underlie human vision and plant photosynthesis. The light that excites the human visual system is a very small portion of the electromagnetic spectrum. A rainbow shows the optical (visible) part of the electromagnetic spectrum; infrared (if it could be seen) would be located just beyond the red side of the rainbow whilst ultraviolet would appear just beyond the opposite violet end.


title: Electromagnetic spectrum

text: Electromagnetic radiation with a wavelength between 380 nm and 760 nm (400–790 terahertz) is detected by the human eye and perceived as visible light. Other wavelengths, especially near infrared (longer than 760 nm) and ultraviolet (shorter than 380 nm) are also sometimes referred to as light, especially when the visibility to humans is not relevant. White light is a combination of lights of different wavelengths in the visible spectrum. Passing white light through a prism splits it up into the several colours of light observed in the visible spectrum between 400 nm and 780 nm.


title: Electromagnetic spectrum

text: If radiation having a frequency in the visible region of the EM spectrum reflects off an object, say, a bowl of fruit, and then strikes the eyes, this results in visual perception of the scene. The brain's visual system processes the multitude of reflected frequencies into different shades and hues, and through this insufficiently-understood psychophysical phenomenon, most people perceive a bowl of fruit.


title: Electromagnetic spectrum

text: At most wavelengths, however, the information carried by electromagnetic radiation is not directly detected by human senses. Natural sources produce EM radiation across the spectrum, and technology can also manipulate a broad range of wavelengths. Optical fiber transmits light that, although not necessarily in the visible part of the spectrum (it is usually infrared), can carry information. The modulation is similar to that used with radio waves.


title: Electromagnetic spectrum

text: Next in frequency comes ultraviolet (UV). The wavelength of UV rays is shorter than the violet end of the visible spectrum but longer than the X-ray.


title: Electromagnetic spectrum

text: UV is the longest wavelength radiation whose photons are energetic enough to ionize atoms, separating electrons from them, and thus causing chemical reactions. Short wavelength UV and the shorter wavelength radiation above it (X-rays and gamma rays) are called "ionizing radiation", and exposure to them can damage living tissue, making them a health hazard. UV can also cause many substances to glow with visible light; this is called "fluorescence".


title: Electromagnetic spectrum

text: At the middle range of UV, UV rays cannot ionize but can break chemical bonds, making molecules unusually reactive. Sunburn, for example, is caused by the disruptive effects of middle range UV radiation on skin cells, which is the main cause of skin cancer. UV rays in the middle range can irreparably damage the complex DNA molecules in the cells producing thymine dimers making it a very potent mutagen.


title: Electromagnetic spectrum

text: The Sun emits significant UV radiation (about 10% of its total power), including extremely short wavelength UV that could potentially destroy most life on land (ocean water would provide some protection for life there). However, most of the Sun's damaging UV wavelengths are absorbed by the atmosphere before they reach the surface. The higher energy (shortest wavelength) ranges of UV (called "vacuum UV") are absorbed by nitrogen and, at longer wavelengths, by simple diatomic oxygen in the air. Most of the UV in the mid-range of energy is blocked by the ozone layer, which absorbs strongly in the important 200–315 nm range, the lower energy part of which is too long for ordinary dioxygen in air to absorb. This leaves less than 3% of sunlight at sea level in UV, with all of this remainder at the lower energies. The remainder is UV-A, along with some UV-B. The very lowest energy range of UV between 315 nm and visible light (called UV-A) is not blocked well by the atmosphere, but does not cause sunburn and does less biological damage. However, it is not harmless and does create oxygen radicals, mutations and skin damage.


title: Electromagnetic spectrum

text: After UV come X-rays, which, like the upper ranges of UV are also ionizing. However, due to their higher energies, X-rays can also interact with matter by means of the Compton effect. Hard X-rays have shorter wavelengths than soft X-rays and as they can pass through many substances with little absorption, they can be used to 'see through' objects with 'thicknesses' less than that equivalent to a few meters of water. One notable use is diagnostic X-ray imaging in medicine (a process known as radiography). X-rays are useful as probes in high-energy physics. In astronomy, the accretion disks around neutron stars and black holes emit X-rays, enabling studies of these phenomena. X-rays are also emitted by stellar corona and are strongly emitted by some types of nebulae. However, X-ray telescopes must be placed outside the Earth's atmosphere to see astronomical X-rays, since the great depth of the atmosphere of Earth is opaque to X-rays (with areal density of 1000 g/cm), equivalent to 10 meters thickness of water. This is an amount sufficient to block almost all astronomical X-rays (and also astronomical gamma rays—see below).


title: Electromagnetic spectrum

text: After hard X-rays come gamma rays, which were discovered by Paul Ulrich Villard in 1900. These are the most energetic photons, having no defined lower limit to their wavelength. In astronomy they are valuable for studying high-energy objects or regions, however as with X-rays this can only be done with telescopes outside the Earth's atmosphere. Gamma rays are used experimentally by physicists for their penetrating ability and are produced by a number of radioisotopes. They are used for irradiation of foods and seeds for sterilization, and in medicine they are occasionally used in radiation cancer therapy. More commonly, gamma rays are used for diagnostic imaging in nuclear medicine, an example being PET scans. The wavelength of gamma rays can be measured with high accuracy through the effects of Compton scattering.


title: Republic of the Congo

text: The Republic of the Congo (, ), also known as Congo-Brazzaville, the Congo Republic or simply either Congo or the Congo, is a country located in the western coast of Central Africa to the west of the Congo river. It is bordered to the west by Gabon, to its northwest by Cameroon and its northeast by the Central African Republic, to the southeast by the Democratic Republic of the Congo, to its south by the Angolan exclave of Cabinda and to its southwest by the Atlantic Ocean.


title: Republic of the Congo

text: The region was dominated by Bantu-speaking tribes at least 3,000 years ago, who built trade links leading into the Congo River basin. Congo was formerly part of the French colony of Equatorial Africa. The Republic of the Congo was established on 28 November 1958 and gained independence from France in 1960. It was a Marxist–Leninist state from 1969 to 1992, under the name People's Republic of the Congo. The country has had multi-party elections since 1992, but a democratically elected government was ousted in the 1997 Republic of the Congo Civil War. President Denis Sassou Nguesso who first came to power in 1979 ruled until 1992 and then again from 1997 onwards.


title: Republic of the Congo

text: It is a member of the African Union, the United Nations, La Francophonie, the Economic Community of Central African States, and the Non-Aligned Movement. It has become the 4th-largest oil producer in the Gulf of Guinea, providing the country with a degree of prosperity, with political and economic instability in some areas and unequal distribution of oil revenue nationwide. Its economy is dependent on the oil sector, and economic growth has slowed since the post-2015 drop in oil prices. With a population of 5.2 million, 88.5% of the country practices Christianity.


title: Republic of the Congo

text: It is named after the Congo River whose name is derived from Kongo, a Bantu kingdom which occupied its mouth around the time the Portuguese first arrived in 1483 or 1484. The kingdom's name derived from its people, the Bakongo, an endonym said to mean "hunters" (, "nkongo").


title: Republic of the Congo

text: During the period when it was colonized by France, it was known as the "French Congo" or "Middle Congo". To distinguish it from the neighboring Democratic Republic of the Congo, it is sometimes referred to as "Congo (Brazzaville)" or "Congo-Brazzaville". Brazzaville derives from the colony's founder, Pierre Savorgnan de Brazzà, an Italian nobleman whose title referred to the town of Brazzacco, in the comune of Moruzzo, whose name derived from the Latin "Brattius" or "Braccius", both meaning "arm".


title: Republic of the Congo

text: Bantu-speaking peoples who founded tribes during the Bantu expansions mostly displaced and absorbed the earlier inhabitants of the region, the Pygmy people, about 1500BC. The Bakongo, a Bantu ethnic group that occupied parts of what later is Angola, Gabon, and the Democratic Republic of the Congo, formed the basis for ethnic affinities and rivalries among those countries. Some Bantu kingdoms—including those of the Kongo, the Loango, and the Teke—built trade links leading into the Congo River basin.


title: Republic of the Congo

text: The Portuguese explorer Diogo Cão reached the mouth of the Congo in 1484. Commercial relationships grew between the inland Bantu kingdoms and European merchants who traded in commodities, manufactured goods, and people captured and enslaved in the hinterlands. After centuries as a central hub for transatlantic trade, direct European colonization of the Congo river delta began in the 19th century, subsequently eroding the power of the Bantu societies in the region.


title: Republic of the Congo

text: The area north of the Congo River came under French sovereignty in 1880 as a result of Pierre de Brazza's treaty with King Makoko of the Bateke. After the death of Makoko, his widow Queen Ngalifourou upheld the terms of the treaty and became an ally to the colonizers. This Congo Colony became known first as French Congo, then as Middle Congo in 1903.


title: Republic of the Congo

text: In 1908, France organized French Equatorial Africa (AEF), comprising the Middle Congo, Gabon, Chad, and Oubangui-Chari (what later is Central African Republic). The French designated Brazzaville as the federal capital. Economic development during the first 50 years of colonial rule in Congo centered on natural-resource extraction. Construction of the Congo-Ocean Railway following World War I has been estimated to have cost at least 14,000 lives.


title: Republic of the Congo

text: During the Nazi occupation of France during World War II, Brazzaville functioned as the symbolic capital of Free France between 1940 and 1943. The Brazzaville Conference of 1944 heralded a period of reform in French colonial policy. Congo "benefited" from the postwar expansion of colonial administrative and infrastructure spending as a result of its central geographic location within AEF and the federal capital at Brazzaville. It had a local legislature after the adoption of the 1946 constitution that established the Fourth Republic.