Proteobacteria

Caulobacterales
   Parvularculales
   Rhizobiales - e.g. Rhizobium
   Rhodobacterales
   Rhodospirillales - e.g. Acetobacter
   Rickettsiales - e.g. Rickettsia
   Sphingomonadales
Beta Proteobacteria
   Burkholderiales
   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. Coxiella
   Methylococcales
   Oceanospirillales
   Pasteurellales
   Pseudomonadales - e.g. Pseudomonas
   Thiotrichales - e.g. Thiomargarita
   Vibrionales - e.g. Vibrio
   Xanthomonadales - e.g. Stenotrophomonas
Delta Proteobacteria
   Bdellovibrionales - e.g. Bdellovibrio
   Desulfobacterales
   Desulfovibrinoales
   Desulfurellales
   Desulfuromonadales
   Myxococcales - myxobacteria
   Syntrophobacterales
Epsilon Proteobacteria
   Campylobacterales - e.g. Helicobacter
   Nautiliales

</table> 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 RNA sequences, and is named for the Greek god Proteus, who could change his shape, because of the great diversity of forms found in it. All proteobacteria are Gram-negative, with a cell wall formed mainly of lipopolysaccharides. Many move about using flagella, but some rely on bacterial gliding. These 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 proteobacteria are anaerobic, but there are many exceptions. The mitochondria that allow eukaryotic cells to conduct aerobic repiration are derived from proteobacteria, probably similar to rickettsias. Nutrition is usually heterotrophic, but there are two groups that conduct photosynthesis, called purple bacteria. The purple sulfur bacteria use sulfur or hydrogen sulfide as an electron donor, and the purple non-sulfur bacteria use hydrogen. Because this role is not played by water, as it is among plants and cyanobacteria, no oxygen is produced. The proteobacteria are divided into five sections, referred to by the Greek letters alpha through epsilon, again based on RNA sequences. Some may be paraphyletic. These are often treated as classes. The currently recognized orders are listed at right, together with some representative genera.