The Rickettsiales, informally called rickettsias, are an order of small Alphaproteobacteria. They are obligate intracellular parasites, and some are notable pathogens, including Rickettsia, which causes a variety of diseases in humans, and Ehrlichia, which causes diseases in livestock. Another genus of well-known Rickettsiales is the Wolbachia, which infect about two-thirds of all arthropods and nearly all filarial nematodes.^[2] Genetic studies support the endosymbiotic theory according to which mitochondria and related organelles developed from members of this group.^[3]

Quick Facts Scientific classification, Families ...

Rickettsiales
Rickettsia rickettsii (red dots) in the cell of a deer tick
Scientific classification
Domain:	Bacteria
Phylum:	Pseudomonadota
Class:	Alphaproteobacteria
Order:	Rickettsiales Gieszczykiewicz 1939 (Approved Lists 1980)
Families
"Candidatus Deianiraeaceae" Castelli et al. 2019 Ehrlichiaceae Moshkovski 1945 (Approved Lists 1980) "Candidatus Midichloriaceae" Montagna et al. 2013 Rickettsiaceae Pinkerton 1936 (Approved Lists 1980) "Candidatus Tenuibacteraceae" Kroer et al. 2016 Genera incertae sedis "Candidatus Anadelfobacter veles" Vannini et al. 2010 "Candidatus Repentibacter" corrig. Prokopchuk et al. 2019 "Sinorickettsia" Li et al. 2021[1]

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The Rickettsiales are difficult to culture, as they rely on living eukaryotic host cells for their survival.

The Rickettsiales further consist of three known families, the Rickettsiaceae, the Midichloriaceae, and the Ehrlichiaceae. Most studies also support the inclusion of the Holosporaceae, but one study has challenged this view.^[4] In that alternative, the Holosporaceae are the sole representatives of their own order, the Holosporales, and as such not part of the Rickettsiales (see the schematic tree below). Other lineages, not clearly part of any family, have been described, as well. Examples include Candidatus Arcanobacter lacustris ^[5] and Rickettsiales bacterium Ac37b.

Rickettsiales genomes are undergoing reductive evolution and are typically small (generally < 1,5 Mbp), AT-rich (generally < 40% GC) with a low coding density (generally < 85%) and a relatively high number of pseudogenes.^[11] Reduction in genome size, % GC and coding density and genes are generally attributed to genetic drift and Muller's ratchet. Genetic drift is enhanced in Rickettsiales genomes due to low population sizes (given their endosymbiotic nature) and frequent population bottlenecks. Similarly, Muller's ratchet is activated through the lack of recombination and horizontal gene transfer (the eukaryotic host cell is a natural barrier).