Alpha Proteobacteria
   Caulobacterales - e.g. Caulobacter
   Parvularculales
   Rhizobiales - e.g. rhizobia
   Rhodobacterales
   Rhodospirillales - e.g. Acetobacter
   Rickettsiales - e.g. Rickettsia
   Sphingomonadales - e.g. Sphingomonas
Beta Proteobacteria
   Burkholderiales - e.g. Bordetella
   Hydrogenophilales
   Methylophilales
   Neisseriales - e.g. Neisseria
   Nitrosomonadales
   Rhodocyclales
   Procabacteriales
Gamma Proteobacteria
   Acidithiobacillales
   Aeromonadales - e.g. Aeromonas
   Alteromonadales - e.g. Pseudoalteromonas
   Cardiobacteriales
   Chromatiales - purple sulfur bacteria
   Enterobacteriales - e.g. Escherichia
   Legionellales - e.g. Legionella
   Methylococcales
   Oceanospirillales
   Pasteurellales - e.g. Haemophilus
   Pseudomonadales - e.g. Pseudomonas
   Thiotrichales - e.g. Thiomargarita
   Vibrionales - e.g. Vibrio
   Xanthomonadales - e.g. Stenotrophomonas
Delta Proteobacteria
   Bdellovibrionales - e.g. Bdellovibrio
   Desulfobacterales
   Desulfovibrionales
   Desulfurellales
   Desulfuromonadales - e.g. Geobacter
   Myxococcales - myxobacteria
   Syntrophobacterales
Epsilon Proteobacteria
   Campylobacterales - e.g. Helicobacter
   Nautiliales
The Proteobacteria are a major group of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera. Others are free-living, and include many of the bacteria responsible for nitrogen fixation. The group is defined mainly in terms of ribosomal RNA sequences, and is named for the Greek god Proteus (also the name of a bacterial genus within the Proteobacteria), who could change his shape, because of the great diversity of forms found in it.

All Proteobacteria are Gram-negative, with an outer membrane mainly composed of liposaccharides . Many move about using flagella, but some are non-motile or rely on bacterial gliding. The last include the myxobacteria, a unique group of bacteria that can aggregate to form multicellular fruiting bodies. There is also a wide variety in the types of metabolism. Most members are facultatively or obligately anaerobic and heterotrophic, but there are numerous exceptions. A variety of genera, which are not closely related, are conserting energy from light through photosynthesis. These are called purple bacteria, referring to their mostly reddish pigmentation.

The proteobacteria are divided into five sections, referred to by the Greek letters alpha through epsilon, again based on rRNA sequences. These are often treated as classes. Some may be paraphyletic.

The Alphaproteobacteria comprise most phototrophic genera, but also several genera metabolising C1-compounds, symbionts of plants (e. g. the rhizobia) and animals and a group of dangerous pathogens, the Rickettsiaceae. Moreover, the precursors of the mitochondria of eukaryotic cells are thought to have originated in this bacterial group. (See endosymbiotic theory.)

The Betaproteobacteria consist of several groups of aerobic of facultative bacteria which are often highly versatile in their degradation capacities, but also contain chemolithotrophic genera (e. g. the ammonia-oxidising genus Nitrosomonas) and some phototrophs (genera Rhodocyclus and Rubrivivax). Many of them are found in environmental samples, such as waste water or soil. Pathogenic species within this class are the Neisseriaceae (gonorrhoe and meningoencephalitis) and species of the genus Burkholderia.

The Gammaproteobacteria comprise several medically and scientifically important groups of bacteria, such as the Enterobacteriaceae, Vibrionaceae and Pseudomonadaceae. A exceeding number of important pathogens belongs to this class, e. g. Salmonella (enteritis and typoid fever), Yersinia (plague), Vibrio (cholera), Pseudomonas aeruginosa (lung infections in hospitalised or cystic fibrosis patients).

The Deltaproteobacteria comprise a branch of predominantly aerobic genera, the fruiting-body-forming myxobacteria, and a branch of strictly anaerobic genera, which contains most of the known sulfate- (Desulfovibrio, Desulfobacter, Desulfococcus, Desulfonema, etc.) and sulfur-reducing bacteria (e. g. Desulfuromonas) alongside several other anaerobic bacteria with different physiology (e. g. ferric iron-reducing Geobacter and syntrophic Pelobacter and Syntrophus species).

The Epsilonproteobacteria consist of only a few genera, mainly the curved to spirilloid Wolinella , Helicobacter, and Campylobacter. They all inhabit the digestive tract of animals and humans and serve as symbionts (Wolinella in cows) or pathogens (Helicobacter in the stomach, Campylobacter in the duodenum).

Vibrio cholerae, γ subdivision

Helicobacter pylori, ε subdivision