The Proteobacteria are a major group (phylum) of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera.^[1] Others are free-living, and include many of the bacteria responsible for nitrogen fixation. The group is defined primarily in terms of ribosomal RNA (rRNA) 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 this group.^[2]

All Proteobacteria are Gram-negative, with an outer membrane mainly composed of lipopolysaccharides. 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 to each other, convert energy from light through photosynthesis. These are called purple bacteria, referring to their mostly reddish pigmentation.

Cavalier-Smith has postulated that Proteobacteria are part of the bacterial clade Gracilicutes.

Sections/Classes

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. Although it has been suggested previously that the Gammaproteobacteria are paraphyletic to the Betaproteobacteria, recent molecular data suggests that this is not so. The divisions of the proteobacteria were once regarded as subclasses (e.g. ?-subclass of the Proteobacteria), but are now regarded as classes (e.g. the Alphaproteobacteria) and should be styled in italics as one word^[3].

Alphaproteobacteria

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

Betaproteobacteria

The Betaproteobacteria consist of several groups of aerobic or 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 (members of the genera Rhodocyclus and Rubrivivax). Betaproteobacteria play an 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. Pathogenic species within this class are the Neisseriaceae (gonorrhea and meningitis) and species of the genus Burkholderia.

Gammaproteobacteria

Vibrio cholerae

Deltaproteobacteria

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 spp.) alongside several other anaerobic bacteria with different physiology (e.g. ferric iron-reducing Geobacter spp. and syntrophic Pelobacter and Syntrophus spp.).

Epsilonproteobacteria

Helicobacter pylori Helicobacter pylori

References

External links

• Proteobacteria information from [[Palaeos]]

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