Rickettsiae are Gram-negative bacteria which multiply only inside host cells and need
arthropods either as reservoirs or as vectors. Using the polymerase chain reaction
and an automated laser fluorescent DNA sequencer, we amplified and sequenced the 16S
rRNA (rDNA) of all available bacteria of the genus Rickettsia. R. tsutsugamushi remained
close to the other bacteria of the genus Rickettsia using this technique, contrary
to previous conclusions based on the study of the Sta-58 protein antigen. We found
that R. canada was not included in the typhus group, as is currently recognized, but
was grouped with the rickettsiae of the spotted fever group (SFG). All the SFG rickettsiae
tested were grouped in the same cluster (R. conorii, Indian tick typhus rickettsia,
Astrakhan fever rickettsia, Israeli tick typhus rickettsia, HA-91, R. sibirica, R.
parkeri, "R. africae", "R. slovaca", R. rickettsii, Thai tick typhus rickettsia, R.
japonica, R. massiliae, R. rhipicephali, R. montana, two recent isolates GS and Bar
29, R. australis, R. akari, R. bellii and R. helvetica). The recently described ELB
bacterium, the agent of the Californian murine typhus, and AB bacterium, a bacterium
associated with male killing in the ladybird beetle, were found in this cluster. The
sequences of R. conorii Moroccan strain/Indian tick typhus rickettsia, R. massiliae/GS
and R. sibirica/HA-91 were identical. All the rickettsiae had a unique ancestor with
bacteria also isolated in arthropods (Ehrlichia, Cowdria, Anaplasma, Wolbachia pipientis),
eventually pathogenic for mammals and implicated in parthenogenesis and cytoplasmic
incompatibility. We conclude that a unique bacterium started a stable association
with arthropod ancestors and generated the observed diversity of the currently isolated
members of the Rickettsiales.