The Proteobacteria are a major group (phylum) of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and Yersinia, and many other notable genera. Others are free-living (nonparasitic), and include many of the bacteria responsible for nitrogen fixation.
Carl Woese established this grouping in 1987, calling it informally the "purple bacteria and their relatives". Because of the great diversity of forms found in this group, the Proteobacteria are named after Proteus, a Greek god of the sea capable of assuming many different shapes; it is not named after the genus Proteus.
Alphaproteobacteria grow at very low levels of nutrients and have unusual morphology such as stalks and buds. They include agriculturally important bacteria capable of inducing nitrogen fixation in symbiosis with plants. An example of alphaproteobacteria is Wolbachia which is the most common infectious bacterial genus in the world that lives only inside the cells of their hosts, usually insects.
Betaproteobacteria often use nutrient substances that diffuse away from areas of anaerobic decomposition of organic matter (hydrogen gas, ammonia, methane) and includes chemoautotrophs. An example of betaproteobacteria is Bordetella pertussis which causes pertussis, or whooping cough.
Gammaproteobacteria are the largest subgroup which include Acinetobacter, Pseudomonas, Escherichia coli, Salmonella, and Serratia marcescens.
Deltaproteobacteria include bacteria that are predators on other bacteria and are important contributors to the sulfur cycle. An example is Desulfovibrio which is found in anaerobic sediments and in the intestinal tracts of humans and animals.
Epsilonproteobacteria are slender Gram-negative rods that are helical or curved. They are also motile by flagella and are microaerophilic. An example is Helicobacter which has been identified as the most common cause of peptic ulcers in humans and a cause of stomach cancer.
Characteristics
All proteobacteria are Gram-negative, with an outer membrane mainly composed of lipopolysaccharides. Many move about using flagella, but some are nonmotile or rely on bacterial gliding. The last include the myxobacteria, a unique group of bacteria that can aggregate to form multicellular fruiting bodies. Also a wide variety in the types of metabolism exists. Most members are facultatively or obligately anaerobic, chemoautotrophs, and heterotrophic, but numerous exceptions occur. A variety of genera, which are not closely related to each other, convert energy from light through photosynthesis. These are called purple bacteria, referring to their mostly reddish pigmentation.
Taxonomy
The group is defined primarily in terms of ribosomal RNA (rRNA) sequences. The Proteobacteria are divided into six sections, referred to by the Greek letters alpha through zeta. These were previously regarded as subclasses of the phylum, but they are now treated as classes. The alpha, beta, delta, and epsilon classes are monophyletic. The genus Acidithiobacillus, part of the Gammaproteobacteria until it was transferred to Class Acidithiobacillia in 2013, is paraphyletic to Betaproteobacteria according to multigenome alignment studies.
References
External links
- Proteobacteria information from Palaeos.
- Proteobacteria. â€" J. P. Euzéby: List of Prokaryotic names with Standing in Nomenclature.
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