Class δ-proteobacteria

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tree

  • Class δ-proteobacteria
    • Order Bdellovibrionales
      • Family Bdellovibrionaceae (e.g. Bdellovibrio, Vampirovibrio)
      • Family Bacteriovoracaceae (e.g. Bacteriovorax, Peredibacter)
    • Order Desulfarcales
      • Family Desulfarculaceae (Desulfarculus)
    • Order Desulfovibrionales
      • Family Desulfovibrionaceae (e.g. Desulfovibrio)
      • Family Desulfomicrobiaceae (e.g. Desulfomicrobium)
      • Family Desulfohalobiaceae (e.g. Desulfohalobium, Desulfomonas)
      • Family Desulfonatronumaceae (Desulfonatronum)
    • Order Desulfobacterales
      • Family Desulfobacteraceae (e.g. Desulfobacter, Desulfonema)
      • Family Desulfobulbaceae (e.g. Desulfobulbus, Desulfocapsa)
      • Family Nitrospinaceae (Nitrospina)
    • Order Desulfurellales
      • Family Desulfurellaceae (Desulfurella, Hippea)
    • Order Desulfuromonales
      • Family Desulfuromonaceae (e.g. Desulforomonas, Pelobacter)
      • Family Geobacteraceae (e.g. Geobacter, Trichlorobacter)
    • Order Myxococcales
      • Family Cystobacterineae (e.g. Cystobacter, Stigmatella)
      • Family Myxococcaceae (e.g. Angiococcus, Myxococcus)
      • Family Polyangiaceae (e.g. Chondromyces, Polyangium)
      • Family Nannocystineae (e.g. Nannocyctis)
      • Family Haliangiaceae (Haliangium)
      • Family Kofleriaceae (Kofleria)
    • Order Syntrophobacterales
      • Family Syntrophobacteraceae (e.g. Desulforhabdus, Syntrophobacter)
      • Family Syntrophaceae (e.g. Desulfomonile, Syntrophus)

About this Class

Diversity

Most of the δ-proteobacteria are sulfate reducers, and this is probably the primitive phenotype of the group. This being also true of the ε-proteobacteria suggests that these two groups share a common ancestral sulfate-reducing phenotype, either because they are specifically related (share a common branch off of the remainder of the proteobacterial tree) or because sulfate reduction is the primitive phenotype of the proteobacteria in general.

Metabolism

Although a much smaller group than the α-, β- or γ-proteobacteria, like these other groups the metabolisms found in the δ-proteobacteria is very broad. Characterized members of this group fall into three general classes of metabolism; anaerobic sulfate reducers and syntrophic hydrogen-generating heterotrophs, aerobic heterotrophs, and parasites of other Bacteria.

Morphology

Most of the organisms is this group are straight or slightly curved rods.

Habitat

The habitats of these organisms varies with their phenotypes. The sulfate reducers are the most common, and are sometimes predominant members of sulfate-containing anaerobic environments.

Sulfate reducers & hydrogenic syntrophs

Sulfate-reducers are common in many anaerobic environments, but are predominant in marine and estuary sediments (saltwater is rich in sulfate). Metabolism is anaerobic respiration, using organics (or sometimes H2) as the electron donor for electron transport. Sulfate is the terminal electron acceptor. In the environment, this sulfide often reacts with metal cations (chiefly Fe+++) to produce insoluble black metal sulfides. This is the black color typical of marine, estuary, and nutrient-rich freshwater sediments and muds.

Metabolism by the hydrogenic syntrophs is related to that of the sulfate reducers, except that protons are used as the terminal electron acceptor in place of sulfate, to generate molecular hydrogen. This process is energetically favorable only if the ambient concentration of hydrogen is vanishingly low, and so they must live in symbiosis with hydrogen-utilizing organisms, such as methanogens, that consume the hydrogen as quickly as it is generated.

Example: Desulfovibrio desulfuricans

Desulfovibrio
Desulfovibrio desulfuricans : Judy Wall, http://www.lbl.gov/Publications/Currents/Archive/Apr-30-2004.html

D. desulfuricans are slightly curved rods, motile by a single polar flagellum. Oxidizes hydrogen and a wide range of organics, including glycerol (which most anaerobes cannot utilize). Organics are incompletely oxidized, generating acetate as the primary waste product. When oxidizing hydrogen, they require acetate as a source of carbon for growth; they are not autotrophic. D. desulfuricans can reduce sulfate or sulfite, or even protons; hydrogenic growth requires removal of this hydrogen product, either by thorough flushing of the media or co-cultivation with methanogens (i.e. syntrophically).

Myxobacteria

The myxobacteria are unicellular aerobic gliders (twitching motility is also used) with a complex life cycle, usually found in terrestrial organic-rich environments, especially on bark or decomposing leaves and wood. They grow individually in thin swarming sheets, excreting lytic and digestive enzymes that lyse other Bacteria, on which the myxobacteria feed. When starved, myxobacteria aggregate and develop into fruiting bodies, with base, stalk and spore cells. This is a terminal differentiation; only the spore cells have a future, and so the fruiting body is a true multicellular organism. Spores are released into the environment, & those that blow or wash to a better environment germinate to produce a new crop of free-living cells.

Example: Myxococcus xanthus

Myxococcus
Myxococcus xanthus : Photo by Michel Vos, 2005

M. xanthus is the best-studied member of this group, being the easiest to grow to high density in liquid cultures and being genetically manipulatable. As a result, aggregation and sporangium formation in M. xanthus are model systems for bacterial cell-cell communication, self-organization, and development. It produces simple spheroid fruiting bodies on short stalks.

Parasites of other Bacteria

Although many bacteria are predatory, that is they can kill others and feed on the nutrients released, the Bdellovibrios actually invade or attach to the surface of other bacterial cells and parasitize them. As a result, they grow as plaques on lawns of host Bacteria, much like the plaques produced by viruses. Members of the genus Bdellovibrio can parasitize a wide range of Gram-negative Bacteria (and a few Gram-positives) and take up residence in the periplasmic space of the host. Others, e.g. Vampirococcus, are very host-specific (infecting only Chromatium, in this example) and are epibiotic, attaching to the surface but not entering into the host.

Example: Bdellovibrio bacteriovorans

Bdellovibrio
Bdellovibrio bacteriovorans life cycle. B. bacteriovorans in yellow, host cell in blue.
Max Planck Institute for Developmental Biology/Rendulic, Berger and Schuster

B. bacteriovorans is probably the best-studied member of this group. Commonly found in soil and freshwater environments, the swarmer “attack-phase” of this organism is a small (0.25-0.4 x 1-2μm) vibrio, motile by a single polar flagellum. This flagellum is unusual in being ensheathed by the outer membrane. After attachment of the attack-phase parasite to a host cell, the parasite looses the flagellum, then passes through the outer membrane of the host and resides in the periplasmic space. In some cases, the parasite can reside quiescently in the host, but more often immediately begin to extract nutrients from the host cell for growth. The parasitic cell grows by elongation; the length of the resulting spiral-shaped parasite depends primarily on the initial size of the host. When the host is spent, the parasite divides into a number of attack-phase cells, which are released by lysis of the host.