The Proteobacteria form the largest bacterial clade – well-recognised by molecular data, but short on morphological synapomorphies. They possess a large insertion in RNA polymerase and DnaK. While the genus Proteus is included within Proteobacteria, the division is not named after the genus. Instead, both are named after the Greek shape-changing god Proteus – in the case of Proteobacteria, to reflect the wide range of morphologies covered by the clade. Includes photosynthetic purple sulphur (e.g. Rhodocyclus) and non-sulphur (e.g. Rhodobacter) bacteria, intracellular parasites (e.g. Rickettsia), colonial formers of fruiting bodies (Myxococcales), and a wide range of heterotrophs, such as probably the most well-known bacterium of all, Escherichia coli. Divided by molecular data into five large clades, the α-, β-, γ-, δ-, and ε-proteobacteria. Examples – [Alphaproteobacteria] Rhodobacter, Rhizobium, Rickettsia; [Betaproteobacteria] Neisseria, Spirillum; [Gammaproteobacteria] Pseudomonas, Vibrio, Escherichia; [Deltaproteobacteria] Bdellovibrio, Myxococcus; [Epsilonproteobacteria] Helicobacter. Of the five classes, it seems likely that the α-, β- and γ-proteobacteria form a clade, with the α-proteobacteria sister to the other two (which form the clade Chromatibacteria: Cavalier-Smith 2002).
α - mostly comprising of two major phenotypes: purple non-sulfur bacteria and aerobic bacteriochlorophyll-containing bacteria. They are known for their symbiotic properties with plants.
β- mostly comprising of chemoheterotrophs and chemoautotrophs which derive nutrients from the decomposition of organic material. Betaproteobacteria play a role in nitrogen fixation in various types of plants, oxidizing ammonium to produce nitrite- an important chemical for plant function. Many of them are found in environmental samples, such as waste water or soil.
γ- mostly comprising of facultatively anaerobic and fermentative gram-negative bacteria. Gammaproteobacteria have several medically and scientifically important groups of bacteria. A number of important pathogens belongs to this class, e.g. Salmonella spp., Escherichia coli, etc. Some Gammaproteobacteria are methane oxidizers and many of them are in symbiosis with geothermic ocean vent dwelling animals.
δ- mostly represented by morphologically diverse, anaerobic sulfidogens. Some members of this group are considered bacterial predators, having bacteriolytic properties. Deltaproteobacteria have properties like sulfur-reduction or sulfate-reduction, other anaerobic bacteria have different physiology like iron-reduction.
ε- mostly comprising of chemoorganotrophs. Most of the known species inhabit the digestive tract of humans and animals and serve as symbiont. There have also been numerous environmental sequences of Epsilonproteobacteria recovered from hydrothermal vents and cold seep habitats.
Cavalier-Smith, T. 2002. The neomuran origin of archaebacteria, the negibacterial root of the universal tree and bacterial megaclassification. International Journal of Systematic and Evolutionary Microbiology 52: 7-76.