The bacteria that eventually formed the mitochondrion (an organelle in eukaryotic cells) is believed to have originated from this family.

It contains Gram-negative bacteria, very sensitive to environmental exposure, thus is adapted to obligate intracellular infection.

Rickettsia rickettsii is considered the prototypical infectious organism in the group.

Magnetococcus marinus Holosporales Hyphomicrobiales, Rhodobacteraceae,   Rhodospirillales, Sphingomonadales, etc.

Pelagibacter Subgroups Ib, II, IIIa, IIIb, IV and V Proto-mitochondria Neorickettsia Wolbachia Anaplasma Ehrlichia Midichloria Orientia Rickettsia