The epsilon ( ε-) proteobacteria comprise one of the five Classes within the phylum Proteobacteria. They inhabit a wide variety of ecological niches ranging from gastrointestinal tracts of animals to water reservoirs, sewage, oil-field community and deep-sea hydrothermal vents. A significant proportion of the microbial population in deep-sea hydrothermal vents is comprised of epsilon proteobacteria where, because of their ability to carry out different types of metabolism using a variety of alternate electron donors (e.g. H2, formate, elemental sulfur, sulfide, thiosulfate) and acceptors (e.g. sulfite, elemental sulfur, nitrate), they play important role in carbon, nitrogen and sulfur cycles. Many of the epsilon proteobacterial species are host-associated (Helicobacter, Campylobacter, Wolinella) and comprise important human and animal pathogens. Helicobacter pylori is the causative agent for gastric and peptic ulcers and infections with this and the related species H. hepaticus are important predisposing factors in gastric cancers in humans and liver cancers in rodents. Campylobacter jejuni and C. coli are the most common causes of food-born illnesses such as diarrhea worldwide C. jejuni infection can also lead to the neuromuscular disease Guillain-Barre syndrome. The e-proteobacteria are presently distinguished from other bacteria primarily based their branching in the 16S rRNA trees. No reliable phenotypic or molecular characteristic is presently known that is uniquely shared by these bacteria. In the past few years, genome sequences of several e-proteobacterial species have become available. The analyses of these genomes have led to identification of large numbers of genes/proteins, as well as conserved indels, that are uniquely present in either all-available e-proteobacteria species or certain subgroups among them at different phylogenetic depths. These signature proteins and indels provide novel and powerful means for identification of e-proteobacterial species for discovering biochemical and physiological characteristics that are unique to these bacteria.
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