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]
The genome sequence of Rickettsia prowazekii and the origin of mitochondria by Siv G. E. Andersson and others. 1998, abstract and comparative gene inventories: respiratory-chain and tricarboxylic-acid-cycle genes
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. New to topics? Read the docs here!