<unk>  (; common noun<unk> , singular<unk> m) 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,<unk>  were among the first life forms to appear on Earth, and are present in most of its habitats.<unk>  inhabit soil, water, acidic hot springs, radioactive waste, and the deep biosphere of Earth's crust.<unk>  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;<unk>  are responsible for the putrefaction stage in this process. In the biological communities surrounding hydrothermal vents and cold seeps, extremophile<unk>  provide the nutrients needed to sustain life by converting dissolved compounds, such as hydrogen sulphide and methane, to energy.<unk>  also live in symbiotic and parasitic relationships with plants and animals. Most<unk>  have not been characterised and there are many species that cannot be grown in the laboratory. The study of<unk>  is known as<unk> logy, a branch of microbiology. Humans and most other animals carry millions of<unk> . Most are in the gut, and there are many on the skin. Most of the<unk>  in and on the body are harmless or rendered so by the protective effects of the immune system, though many are beneficial, particularly the ones in the gut. However, several species of<unk>  are pathogenic and cause infectious diseases, including cholera, syphilis, anthrax, leprosy, tuberculosis, tetanus and bubonic plague. The most common fatal<unk> l diseases are respiratory infections. Antibiotics are used to treat<unk> l infections and are also used in farming, making antibiotic resistance a growing problem.<unk>  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. Once regarded as plants constituting the class Schizomycetes ("fission fungi"),<unk>  are now classified as prokaryotes. Unlike cells of animals and other eukaryotes,<unk> l cells do not contain a nucleus and rarely harbour membrane-bound organelles. Although the term<unk>  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<unk>  and Archaea.
Bacteria
Yeast
Virus
Enteric bacteria 
Chyme
Microgenia
Microorganism
Mold