Taxonomic studies of Rickettsiella, Rickettsia, and Chlamydia using genomic DNA

Invertebrate pathogens of the genus Rickettsiella (Order Rickettsiales) undergo a developmental cycle more characteristic of chlamydial organisms (Order Chlamydiales) than of typical rickettsiae. Moreover, among recognized species of Rickettsiella, there is considerable variation in host range and in the ultrastructure and development of the infectious stage, i.e., the elementary body. To begin an analysis of the taxonomic relationships of the invertebrate pathogens belonging to the genus Rickettsiella and of the relationship of these to chlamydiae and other rickettsiae, genomic DNA of representative species was compared with respect to restriction enzyme site polymorphism, DNA-DNA hybridization, G + C ratios, and fine melting profiles. The strains studied included isolates currently classified as Rickettsiella grylli, R. popilliae, R. chironomi, Chlamydia psittaci, C. trachomatis, Coxiella burnetii, and Rickettsia conorii. Reciprocal DNA hybridization studies carried out under high-stringency conditions (65 degrees C) showed homology between R. grylli and R. popilliae isolates but not with the others, including two isolates of R. chironomi from the midge, Chironomus dorsalis, and the scorpion, Buthus occitanus. The R. chironomi isolates shared no detectable homology with each other or with any of the other strains or species. Additionally, no homology was detected between any of the Rickettsiella isolates and the species of Coxiella, Rickettsia, or Chlamydia. Results obtained from the DNA fine melting profiles and G + C ratios corresponded with the results obtained from the DNA hybridization studies. Thus, the present study indicates that, despite similarities in developmental cycles, Rickettsiella and Chlamydia are taxonomically distinct groups appropriately placed in different orders. However, the uniqueness of the two R. chironomi species, both from each other and from the other organisms studied, indicates these could be recognized as distinct species.