Rickettsidae

Alphaproteobacteria
Transmission electron micrograph of Wolbachia within an insect cell. Credit:Public Library of Science / Scott O'Neill
Scientific classification
Domain:	Bacteria
Phylum:	Proteobacteria
Class:	Alphaproteobacteria Garrity et al. 2006
subclasses
Magnetococcidae Rickettsidae Rickettsiales Anaplasmataceae Midichloriaceae Rickettsiaceae Pelagibacterales (subgroups I–V) Caulobacteridae Holosporales Caulobacterales Caulobacteraceae Parvularculales Parvularculaceae Rhizobiales Aurantimonadaceae Bartonellaceae Beijerinckiaceae Bradyrhizobiaceae Brucellaceae Cohaesibacteraceae Hyphomicrobiaceae Methylobacteriaceae Methylocystaceae Phyllobacteriaceae Rhizobiaceae Rhodobiaceae Xanthobacteraceae Rhodobacterales Rhodobacteraceae Rhodospirillales Acetobacteraceae Rhodospirillaceae several unclassified Rhodospirillales Sneathiellales Sneathiellaceae Sphingomonadales Erythrobacteraceae Sphingomonadaceae Kiloniellales Kiloniella, Kopriimonas, Rhodovibrio, and Pelagibius Kordiimonadales Kordiimonas and Rhodothalassium

Alphaproteobacteria is a class of bacteria in the phylum Proteobacteria (See also bacterial taxonomy). Its members are highly diverse and possess few commonalities, but nevertheless share a common ancestor. Like all Proteobacteria, its members are Gram-negative and some of its intracellular parasitic members lack peptidoglycan and are consequently gram variable.

The Alphaproteobacteria is a diverse taxon and comprises several phototrophic genera, several genera metabolising C1-compounds (e.g., Methylobacterium spp.), symbionts of plants (e.g., Rhizobium spp.), endosymbionts of arthropods (Wolbachia) and intracellular pathogens (e.g. Rickettsia). Moreover, the class includes (as an extinct member) the , the bacterium that was engulfed by the eukaryotic ancestor and gave rise to the , which are organelles in eukaryotic cells (See endosymbiotic theory). A species of technological interest is Rhizobium radiobacter (formerly Agrobacterium tumefaciens): scientists often use this species to transfer foreign DNA into plant genomes.Aerobic anoxygenic phototrophic bacteria, such as Pelagibacter ubique, are alphaproteobacteria that are a widely distributed marine plankton that may constitute over 10% of the open ocean microbial community.

There is some disagreement on the phylogeny of the orders, especially for the location of the Pelagibacterales, but overall there is some consensus. This issue stems from the large difference in gene content (e.g. genome streamlining in Pelagibacter ubique) and the large difference in GC-richness between members of several orders. Specifically,Pelagibacterales, Rickettsiales and Holosporales contains species with AT-rich genomes. It has been argued that it could be a case of convergent evolution that would result in an artefactual clustering. However, several studies disagree,. Furthermore, it has been found that the GC-content of ribosomal RNA, the traditional phylogenetic marker, little reflects the GC-content of the genome: for example, members of the Holosporales have a much higher ribosomal GC-content than members of the Pelagibacterales and Rickettsiales, which have similarly low genomic GC-content, because they are more closely related to species with high genomic GC-contents than to members of the latter two orders

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