Gammaproteobacteria is a class of bacteria in the phylum Pseudomonadota (synonym Proteobacteria). It contains about 250 genera, which makes it the most genus-rich taxon of the Prokaryotes. Several medically, ecologically, and scientifically important groups of bacteria belong to this class. It is composed by all Gram-negative microbes and is the most phylogenetically and physiologically diverse class of Proteobacteria.
These microorganisms can live in several terrestrial and marine environments, in which they play various important roles, including extreme environments such as hydrothermal vents. They generally have different shapes rods, curved rods, cocci, spirilla, and filaments and include free living bacteria, biofilm formers, commensals and symbionts, some also have the distinctive trait of being bioluminescent. Metabolisms found in the different genera are very different; there are both aerobic and anaerobic (obligate or facultative) species, chemolithoautotrophics, chemoorganotrophics, photoautotrophs and heterotrophs.
Etymology
The element "gamma" (third letter of the Greek alphabet) indicates that this is Class III in Bergey's Manual of Systematic Bacteriology (Vol. II, page 1). Proteus refers to the Greek sea god who could change his shape. Bacteria (Greek βακτήριον; "rod" "little stick"), in terms of etymological history, refers to Bacillus (rod-shaped bacteria), but in this case is "useful in the interim while the phylogenetic data are being integrated into formal bacterial taxonomy."
Phylogeny
Currently, many different classifications are based on different approaches, such as the National Center for Biotechnology Information, based on genomic, List of Prokaryotic names with Standing in Nomenclature , ARB-Silva Database based on ribosomal RNA, or a multiprotein approach. It is still very difficult to resolve the phylogeny of this bacterial class.
Here, it is reported a clade based on a set of 356 protein families for the class of Gammaproteobacteria.
A number of bacteria have been described as members of the Gammaproteobacteria, but have not yet been assigned an order or family. These include bacteria of the genera Alkalimonas, Gallaecimonas, Ignatzschineria, Litorivivens, Marinicella, Plasticicumulans, Pseudohongiella, Sedimenticola, Thiohalobacter, Thiohalorhabdus, Thiolapillus, and Wohlfahrtiimonas.
Significance and applications
Gammaproteobacteria, especially the orders Alteromonadales and Vibrionales, are fundamental in marine and coastal ecosystems because they are the major groups involved in the nutrients cycling and despite their fame as pathogens, they find application in a huge number of fields, such as bioremediation and biosynthesis.
Gammaproteobacteria can be used as a microbial fuel cell (MFC) element that applies their ability to dissimilate various metals. The produced energy could be collected as one of the most environmentally friendly and sustainable energy production systems. They are also used as biological methane filters.
Phototrophic purple sulfur bacteria are used in wastewater treatment processes and the ability of some Gammaproteobacteria (e.g. the genus Alcanivorax) to bioremediate oil is becoming increasingly important to degrade crude oil after oil spills. Some species from the family Chromatiaceae are notable because might be involved in the production of vitamin B12. Another application of some Gammaproteobacteria is their ability to synthesize Poly-b-hydroxyalkanoate (PHA) which is a polymer that is used in the production of biodegradable plastics. Also lots of Gammaproteobacteria species are able to generate secondary metabolites with antibacterial properties.