Class ε-proteobacteria

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  • Class ε-proteobacteria
    • Order Campylobacteriales
      • Family Campylobacteraceae (e.g. Campylobacter, Sulfuospirillum)
      • Family Helicobacteraceae (e.g. Helicobacter, Sulfuricurvum, Wolinella)
      • Family Hydrogenomonaceae (Hydrogenomonas)
    • Order Nautiales
      • Family Nautiliaceae (e.g. Caminibacter, Nautilia, Thioreductor)

About this Class

Diversity

Familiar ε-proteobacteria form a relatively narrow phylogenetic group of intestinal symbionts, but the phylogenetic range of environmental members of this group, represented by rRNA sequences extracted primarily from deep-sea environments, suggests that this is in actuality a very large and phylogenetically diverse Class.

Metabolism

These organisms are microaerophilic or anaerobic heterotrophs or chemoautotrophs. Metabolism is by anaerobic respiration using a wide range of organic and inorganic electron donors and acceptors, but generally not carbohydrates. A common feature of the ε-proteobacteria is the ability to use hydrogen as an electron donor for electron transport. Sulfate reduction seems also to be common.

Morphology

Most of the cultivated ε-proteobacteria are helically curved rods, and are motile by polar flagella. The deep-sea environmental ε-proteobacteria have a wide range of morphologies, including vibrio and helical forms, but also rods, cocci, and filaments of all types.

Habitat

The most well-known ε-proteobacteria are intestinal symbionts, but in deep sea environments, and particularly marine hydrothermal zones, the ε-proteobacteria are predominant. Deep sea hydrothermal ε-proteobacteria are abundant not just in water and sediment, but as external and endosymbiotic inhabitants of vent-associated animals.

Intestinal symbionts

Members of the genera Campylobacter, Helicobacter and Wolinella are inhabitants of the upper GI epithelium of mammals and birds. Most are commensalistic, at least in their natural host, but some are pathogens and many cause zoonotic disease. For example, Campylobacter is a common commensal in birds, but in humans is perhaps the single most common cause of foodborne disease.

Example: Helicobacter pylori

Helicobacter
Helicobacter pylori : The Prokaryotes, pp 3496 credited to Alan Curry & Dennis Jones

H. pylori is a microaerophilic curved rod with several unipolar flagella that are sheathed, with a distinctive bulb at the distal ends. It is a common symbiont of the stomach and duodenal lining, colonizing about 70% of humans. In most cases no symptoms occur and the symbiosis persists for life. In some cases, however, colonization by H. pylori results in gastritis or peptic ulcers, and is a contributing factor for stomach cancer. However, there is evidence that H. pylori may also help modulate stomach acidity and reduce acid reflux.

Deep sea hydrothermal vent-associated species

Although few have been cultivated, molecular surveys of environmental samples show that ε-proteobacteria are very common, even predominant, in many marine hydrothermal vent environments. These environments usually bring to mind the hyperthermophilic Archaea, but the cold area surrounding the hot vents are oases of life both macroscopically and microscopically. In fact, it is the mixture of hot, reduced geothermal water and cold oxygenated sea water, each by themselves more-or-less at equilibrium, that creates the chemical disequilibria that provide the chemical potential energy that can be harvested by lithotrophic organisms. Except in the hottest regions of these vents zones, molecular phylogenetic analysis suggests, ε-proteobacteria are very abundant. This includes the sediments, surfaces, and waters of these regions, but also the symbionts of the animals that inhabit these regions.

Example: The endosymbiont of the scaly snail Crysomallon squamiferum

scaly snail
The deep-sea scaly snail : from Shana Goffredi, et al 2004, supplemental material

The scaly snail (Crysomallon squamiferum) is a unique animal found only in Indian Ocean hydrothermal vent fields. Instead of an operculum (the other half of the shell than other snails usually use to cover themselves when retracted), the body of the scaly snail is covered in tough, iron sulfide reinforced scale-like plates. More amazing, the scaly snail has only a vestigial digestive tract and radulus (a scrapping tongue). Instead, the scaly snail has very enlarged esophogeal glands filled with ε-proteobacterial endosymbionts. The animal probably absorbs sulfides from the environment through the crawling surface of its foot; these sulfides are brought to the esophogeal glands along with oxygen absorbed in the gills, Here the sulfide-oxidizing ε-proteobacterial endosymbionts use these to generate energy and fix carbon from CO2. In return for a place to live and a supply of resources, the Bacteria provide the snail with some form of nutrition. The snail, then, is a chemoautotrophic animal. This symbiosis is analogous to that of the giant vent tubeworm (Riftia) and several other hydrothermal vent animals. The symbiotic biofilm covering the snail is predominated by a wide range of ε-proteobacteria, and these presumably participate in the iron sulfide mineralization of the hosts scales and shell surface.