PROTEOBACTERIA

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PHYLUM DELTAPROTEOBACTERIA

INTRODUCTION TO THE DELTAPROTEOBACTERIA

These organisms are formed by two major, and very different groups. One group tends to be unicellular and obligate anaerobes. Most of them use sulfate as the terminal electron acceptor and reduce it to sulfide. They occur in association with anoxic, sulfur-rich mud, geothermal springs, and digestive tracts. Geobacter (see Figure A), first isolated from mud in the Potomac River, can pass off its electrons to metals, so NASA is testing it to see if it can be used to make a living battery that gets its energy through organic waste.

The other major clade is made of obligate aerobes (there are no known facultatively anaerobic taxa in this phylum). The bdellovibrios are predatory cells that feed on other bacteria. They have a polar, sheathed flagellum, which allows them to swim at speeds up to 100 cell-lengths per second in their attack phase. After attaching to a bacterial cell, a bdellovibrio enters the intraperiplasmic space where they feed on the host and then reproduce.

The gliding bacteria live in the vegetative state as gliding rods embedded in a polysaccharide slime. At some cue, they begin to aggregate and form a multicellular structure, which forms a fruiting body. Cells in the fruiting body, called a myxosporangium, form dessication-resistant cells that serve as spores (called myxospores). The myxospores may function in dispersal, but likely they function primarily as resting spores.

A. TEM micrograph of Geobacter.

B. TEM micrograph of Bdellovibrio attacking a host bacterial cell.

C. SEM micrograph of Stigmatella forming myxosporangia.

Images taken from:
A: http://science.nasa.gov/headlines/y2004/18may_wastenot.htm
B: http://microgen.ouhsc.edu/b_bacter/fig1.png
C: http://www.zmbh.uni-heidelberg.de/Schairer/2.JPG

SYNOPTIC DESCRIPTION OF THE DELTAPROTEOBACTERIA

The following description comes mainly from Margulis and Schwartz (1998), Barnes (1984), Brock et al. (1994), and Tudge (2000).

I. SYNONYMS: δ-proteobacteria, sulfate-reducing bacteria, bdellovibrios, and the gliding bacteria.

II. PHYLUM CHARACTERISTICS:

A. Structure

Cell Form: Variable; rods, cocci, spirals or irregularly lobed.

Cell Wall: Gram-.

Motility: Non-motile or motile, the myxobacteria move by gliding over surfaces.

B. Physiology

O₂ Tolerance: Strice anaerobes or aerobes. No facultatively anaerobic taxa.

Substrates: They metabolize simple to complex organic compounds as heterotrophs.

Products: Terminal electron acceptors include Oxygen (for the aerobes) to sulfur, sulfate and small organics. Thus, the anoxic taxa typically reduce sulfate to a variety of sulfur compounds. Almost all metabolize organic sources to carbon dioxide.

C. Other: Very diverse group of bacteria.

D. Ecology: Free-living in soils and aquatic sediments.

SYSTEMATICS OF THE DELTAPROTEOBACTERIA

Stackebrandt et al. (1988), using 16S rRNA sequences, defined a seemingly unrelated group of eubacteria as Proteobacteria, the purple bacteria, which they defined as a class that they called Proteobacteria. Within that group, they defined five separate lines, each defined by a Greek letter: α, β, γ, δ, ε. The second edition of Bergey's Manual of Systematic Bacteriology (Garrity et al. 2003) adopted Proteobacteria, but raised it to phylum level with each of the five groups becoming classes. In order to bring the prokaryotes into line with kingdom-level divisions in the eukaryotes, I felt that it was necessary to raise the Proteobacteria to kingdom-level status with each of the five groups also raised to the level of phylum.

The Gammaproteobacteria has two major groups within it: the strict anaerobic sulfate reducers and the aerobic vibrios and gliding bacteria. I took this separation to be at the class-level (classes Anoxydeltabacteriae and Oxydeltabacteriae). The ordinal structure is from Garrity et al. (2003).

HIERARCHICAL CLASSIFICATION OF THE DELTAPROTEOBACTERIA

This is a modification of Garrity et al. (2003). This system has 2 classes.

Organisms diverse in form. Aerobic or anaerobic; facultative anaerobes not known. Most use inorganic electron acceptors. Some aerobic taxa can digest other bacteria.

CLASS ANOXYDELTABACTERIA

Chemoorganotrophic or chemolithotrophic organisms, most of which are strict anaerobes. Mostly unicellular, but some can form cellular aggregates.

ORDER DESULFURELLALES

Motile rods; strictly anaerobic chemoorganotrophs or chemolithotrophs. Elemental sulfur (or polysulfide) is the most common electron acceptor. Cytochromes absent. Metabolize simple organic sources. From geothermal springs, freshwater or marine.

Desulfurella, Hippea

ORDER DESULFOVIBRIONALES

Motlie rods that may be curved. Strict anaerobes; chemoorganotrophic or chemolithotrophic. Sulfate is the most common electron acceptor, reducing it to sulfide. Metabolize simple organic sources. They do contain cytochromes. They have been found in geothermal springs and almost every type of aquatic system, freshwater, marine, treatment systems, digestive tracts.

Desulfovibrio, Bilophila, Lawsonia

Desulfomicrobium, Desulfonatronovibrio, Desulfothermus

Desulfonatronum

ORDER DESULFOBACTERALES

Sulfate-reducing bacteria that are morphologically diverse (rods, cocci, ovals, and curved; some in cellular aggregates). Anaerobic chemoorganotrophs, chemolithoheterotrophs, or chemolithoautotrophs. Most often sulfate is the electron acceptor is reduced to sulfide. Organic compounds like long-chained fatty acids and aromatic hydrocarbons serve as electron donors. They metabolize organic sources to carbon dioxide (complete oxidation) or acetate (incomplete oxidation). Contain cytochromes. From most aquatic environments.

Desulfobacter, Desulfobacterium, Desulfobacula, Desulfobotulus, Desulfocella, Desulfococcus, Desulfofaba, Desulfofrigus, Desulfonema, Desulfosarcina, Desulfospira, Desulfotignum

Desulfobulbus, Desulfocapsa, Desulfofustis, Desulfotalea

Nitrospina

ORDER DESULFARCALES

Sulfate-reducing rods or comma-shaped bacteria. They are similar to members of the Desulfobacteriales but due to a deep branch in the 16S rRNA tree, separated into a separate order.

Desulfarculus

ORDER DESULFUROMONALES

Motile anaerobic rod-shaped bacteria; chemolithoheterotrophs or chemoorganotrophs; from anoxic marine and freshwater habitats.

Desulfuromonas, Desulfuromusa, Malonomonas, Pelobacter

Geobacter, Trichlorobacter

ORDER SYNTROPHOBACTERALES

Most are sulfate-reducers, and oxidize organic substrates completely to carbon dioxide. A few oxidize sources to acetate. From marine geothermal springs. They grow best at neutral pH.

Syntrophobacter, Desulfacinum, Desulforhabdus, Desulfovirga, Thermodesulforhabdus

Syntrophus, Desulfobacca, Desulfomonile, Smithella

CLASS OXYDELTABACTERIA

Intricate life cycles are common. Aerobic.

ORDER BDELLOVIBRIONALES

Often, comma-shaped cells, each with a polar flagellum. They are obligate aerobes. These are predatory motile cells that feed on other bacteria. They have an attack phase in which they are motile and an intracellular phase in which they are intraperiplasmic. The flagellum is sheathed and the cells can swim up to 100 cell lengths per second.

Bdellovibrio, Bacteriovorax, Micavibrio, Vampirovibrio

ORDER MYXOCOCCALES

These are gliding bacteria that have intricate lifecycles. The mycelium-like structure forms fruiting bodies that yield dessication-resistant myxospores.

Myxococcus, Corallococcus, Pyxicoccus

Cystobacter, Archangium, Hyalangium, Melettangium, Stigmatella

Polyangium, Byssophaga, Chondromyces, Haploangium, Jahnia, Sorangium

Nannocystis

Kofleria

This page is maintained by Jack R. Holt. Last revised: 02/05/2008.