Alphaproteobacteria

Alphaproteobacteria are a large class of Gram-negative bacteria with extraordinary variety in lifestyle. The class includes the most abundant ocean bacteria, nitrogen-fixing partners of plants, important pathogens, and the group from which mitochondria descended.

Pelagibacter ubique, of the SAR11 group, may be the most numerous organism in the ocean. Rhizobium and relatives form root nodules on legumes and fix nitrogen, an important step in the nitrogen cycle. Agrobacterium transfers DNA to plants and is a tool of genetic engineering, and Caulobacter is a model for cell development. Others include the purple photosynthetic bacteria.

The class includes Rickettsia, which causes typhus and spotted fevers and is spread by ticks and lice, Brucella, which causes brucellosis, and Bartonella. These live inside host cells. Also in the group is Wolbachia, an insect symbiont that manipulates its hosts and, through cytoplasmic incompatibility, is used to reduce the spread of mosquito-borne viruses.

Gene comparisons place the ancestor of mitochondria among or near the alphaproteobacteria, with Rickettsia as one of the closest living relatives of the organelle. This supports endosymbiotic theory and the view that the bacterial partner was involved in eukaryogenesis. Their cell envelope is typical of Gram-negative bacteria.

• Mitochondria — the organelle descended from this group
• Wolbachia — an insect symbiont in the class
• Nitrogen Fixation — a process carried out by Rhizobium