Proteobacteria

The Proteobacteria are a major group (phylum) of gram-negative 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

Betaproteobacteria are a class of Proteobacteria. Betaproteobacteria are, like all Proteobacteria, Gram-negative.

The Betaproteobacteria consist of several groups of aerobic or facultative bacteria that are often highly versatile in their degradation capacities, but also contain chemolithotrophic genera (e.g., the ammonia-oxidising genus Nitrosomonas) and some phototrophs (members of the genera Rhodocyclus and Rubrivivax). Betaproteobacteria play a role in nitrogen fixation in various types of plants, oxidizing ammonium to produce nitrite – an important chemical for plant function. Many of them are found in environmental samples, such as waste water or soil. Pathogenic species within this class are the Neisseriaceae (gonorrhea and meningitis) and species of the genus Burkholderia.

Phylogeny

The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature and National Center for Biotechnology Information and the phylogeny is based on 16S rRNA-based LTP release 106 by 'The All-Species Living Tree' Project