Rickettsia

Rickettsia is a genus of intracellular bacteria, many associated with arthropods. Some species cause disease in vertebrate hosts, but a genus name does not identify one universal vector, disease or cellular mechanism. Dependence on host cells makes retained and lost metabolic functions especially informative.

The genome of Rickettsia prowazekii contains respiratory and citric-acid-cycle machinery alongside extensive biosynthetic losses. Comparisons with mitochondrial gene sets helped investigate the bacterial ancestry of mitochondria. ^[ref] Shared features and a reconstructed relationship are evidence about evolutionary history; they do not make a living rickettsial species the unchanged ancestor of every modern mitochondrion.

Rickettsia parkeri is a spotted-fever-group intracellular bacterium used to study Sca2-dependent actin assembly. Its motility machinery should not be generalized to every rickettsial species.

Rickettsia prowazekii is an intracellular bacterium with extensive biosynthetic losses but retained respiratory and citric-acid-cycle functions. Its cytosolic niche and metabolic inventory differ from those of inclusion-resident Chlamydia.