The Epsilonproteobacteria consist of few known genera, mainly the curved to spirilloid Wolinella spp., Helicobacter spp., and Campylobacter spp. Most of the known species inhabit the digestive tracts of animals and serve as symbionts (Wolinella spp. in cattle) or pathogens (Helicobacter spp. in the stomach, Campylobacter spp. in the duodenum).
The Epsilonproteobacteria found at deep-sea hydrothermal vents characteristically exhibit chemolithotrophy, meeting their energy needs by oxidixing reduced sulfur, formate, or hydrogen coupled to the reduction of nitrate or oxygen.[7] Autotrophic Epsilonproteobacteria use the reverse Krebs cycle to fix carbon dioxide into biomass, a pathway originally thought to be of little environmental significance. The oxygen sensitivity of this pathway is consistent with their microaerophilic or anaerobic niche in these environments, and their likely evolution in the Mesoproterozoic oceans,[8] which are thought to have been sulfidic with low levels of oxygen available from cyanobacterial photosynthesis.[9]
Arcobacter Vandamme et al.1991 emend. Vandamme et al.1992
Sulfurospirillum Schumacher et al.1993 emend. Luijten et al.2003
Campylobacter Sebald and Véron 1963 emend. Vandamme et al.2010
Notes:
Prokaryotes where no pure (axenic) asses are isolated or available, i.e. not cultivated or can not be sustained in culture for more than a few serial passages