Alphaproteobacteria[3]
Betaproteobacteria[4]
Gammaproteobacteria[5]
Deltaproteobacteria[6]
Epsilonproteobacteria[7]
Zetaproteobacteria[8]
Acidithiobacillia[5]
Hydrogenophilalia[4]
Oligoflexia[9]
"Candidatus lambdaproteobacteria"
"Candidatus muproteobacteria"
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.[10] 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".[11] 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.[1][12]
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.[13]
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.[14]