Proteobacteria

Proteobacteria is a major phylum of Gram-negative bacteria. They include a wide variety of pathogenic genera, such as "Escherichia", "Salmonella", "Vibrio", "Helicobacter", "Yersinia", "Legionellales", and many others. Others are free-living (nonparasitic) and include many of the bacteria responsible for nitrogen fixation.
Carl Woese established this grouping in 1987, calling it informally the "purple bacteria and their relatives". Because of the great diversity of forms found in this group, it was named after Proteus, a Greek god of the sea capable of assuming many different shapes and is not named after the Proteobacteria genus "Proteus".
Characteristics.
All "Proteobacteria" are Gram-negative (though some may stain Gram-positive or Gram-variable in practice), with an outer membrane mainly composed of lipopolysaccharides. Many move about using flagella, but some are nonmotile or rely on bacterial gliding. The latter include the myxobacteria, an order of bacteria that can aggregate to form multicellular fruiting bodies. Also, a wide variety in the types of metabolism exists. Most members are facultatively or obligately anaerobic, chemolithoautotrophic, and heterotrophic, but numerous exceptions occur. A variety of genera, which are not closely related to each other, convert energy from light through photosynthesis and anoxygenic photosynthesis.
"Proteobacteria" are associated with the imbalance of microbiota of the lower reproductive tract of women. These species are associated with inflammation.
Some Alphaproteobacteria can grow at very low levels of nutrients and have unusual morphology such as stalks and buds. Others include agriculturally important bacteria capable of inducing nitrogen fixation in symbiosis with plants. The type order is the Caulobacterales, comprising stalk-forming bacteria such as "Caulobacter". The mitochondria of eukaryotes are thought to be descendants of an alphaproteobacterium.
The Betaproteobacteria are highly metabolically diverse and contain chemolithoautotrophs, photoautotrophs, and generalist heterotrophs. The type order is the Burkholderiales, comprising an enormous range of metabolic diversity, including opportunistic pathogens.
The Gammaproteobacteria are the largest class in terms of species with validly published names. The type order is the Pseudomonadales, which include the genera "Pseudomonas" and the nitrogen-fixing "Azotobacter".
The Deltaproteobacteria include bacteria that are predators on other bacteria and are important contributors to the anaerobic side of the sulfur cycle. The type order is the Myxococcales, which includes organisms with self-organising abilities such as "Myxococcus" spp.
The Epsilonproteobacteria are often slender, Gram-negative rods that are helical or curved. The type order is the Campylobacterales, which includes important food pathogens such as "Campylobacter" spp.
The Zetaproteobacteria are iron-oxidizing neutrophilic chemolithoautotrophs, distributed worldwide in estuaries and marine habitats. The type order is the Mariprofundales.
The Hydrogenophilalia are obligate thermophiles and include heterotrophs and autotrophs. The type order is the Hydrogenophilales.
The Acidithiobacillia contain only sulfur, iron, and uranium-oxidising autotrophs. The type order is the Acidithiobacillales, which includes economically important organisms used in the mining industry such as "Acidithiobacillus" spp.
The Oligoflexia are filamentous aerobes. The type order is the Oligoflexales, which contains the genus "Oligoflexus".
Taxonomy.
The group is defined primarily in terms of ribosomal RNA (rRNA) sequences. The "Proteobacteria" are divided into nine classes with validly published names, referred to by the Greek letters alpha through zeta, the Acidithiobacillia, Hydrogenophilalia, and Oligoflexia. These were previously regarded as subclasses of the phylum, but they are now treated as classes. 
These classes are monophyletic. The genus "Acidithiobacillus", part of the Gammaproteobacteria until it was transferred to class Acidithiobacillia in 2013, was previously regarded as paraphyletic to the "Betaproteobacteria" according to multigenome alignment studies. In 2017, the Betaproteobacteria was subject to major revisions and the class Hydrogenophilalia was created to contain the order Hydrogenophilales
Proteobacterial classes with validly published names include some prominent genera: e.g.:
Transformation.
Transformation, a process in which genetic material passes from bacterium to another, has been reported in at least 30 species of "Proteobacteria" distributed in the classes alpha, beta, gamma and epsilon. The best-studied "Proteobacteria" with respect to natural genetic transformation are the medically important human pathogens "Neisseria gonorrhoeae" (class beta), "Haemophilus influenzae" (class gamma) and "Helicobacter pylori" (class epsilon). Natural genetic transformation is a sexual process involving DNA transfer from one bacterial cell to another through the intervening medium and the integration of the donor sequence into the recipient genome. In pathogenic "Proteobacteria", transformation appears to serve as a DNA repair process that protects the pathogen's DNA from attack by their host's phagocytic defenses that employ oxidative free radicals.