Keywords

Definition

Proteobacteria is the largest and most diverse phylum of the domain Bacteria.

Overview

Proteobacteria is an evolutionarily, geologically, and environmentally important group of microorganisms. All proteobacteria are Gram-negative bacteria, with an outer membrane mainly composed of lipopolysaccharides. Members of this phylum show extreme metabolic diversity, including chemoautotrophic, chemoorganotrophic, and phototrophic microorganisms, which represent most of the known bacteria of medical, industrial, and agricultural significance. Photosynthetic proteobacteria are called purple bacteria, referring to their reddish pigmentation. Most members of the phylum are facultative or obligate anaerobes, with gas vesicles, flagella, or the ability to move by gliding. Morphologically, they show a variety of cellular forms including rods, curved rods, ovoids, and spirals, and some have stalks or other appendages. There is evidence that eukaryotic mitochondria originated from an endosymbiotic relationship with a member of this group of bacteria. Phylogenetically, the group is defined on the basis of sequence of the small ribosomal subunit RNA gene (16S rRNA) and is divided into five classes: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, and Epsilonproteobacteria.

The Alphaproteobacteria comprise numerous phototrophs, chemolithotrophs, and chemoorganotrophs. Some members of this group are symbionts of plants and animals, and others have adopted intracellular life styles and constitute important human and animal pathogens.

Betaproteobacteria consist of several groups of aerobic or facultative bacteria, which are often highly versatile in their degradation capacities. Many of the species of this class play a role in nitrogen fixation in various types of plants or are able to use ammonium, hydrogen, or methane as a source of energy.

Gammaproteobacteria are the largest subgroup of proteobacteria. Many pathogens belong to this class. Many genera are chemoorganotrophs and facultative anaerobes, while others are chemolithotrophs. Some members of this group are photosynthetic using hydrogen sulfide as environmental reducing power.

Deltaproteobacteria is comprised of chemoorganotrophic microorganisms. This class can be divided into two branches. Aerobic predators and fruiting-body-forming bacteria have been found among one, while most of the known sulfate- and sulfur-reducing bacteria, an activity associated to the sulfur cycle, belong to the other.

Epsilonproteobacteria consist of few known genera with symbionts or pathogens species for humans or animals. Some environmental sequences have been recovered from cold and hydrothermal environments.

See Also