16S RIBOSOMAL-RNA GENE SEQUENCE OF NEORICKETTSIA-HELMINTHOECA AND ITSPHYLOGENETIC ALIGNMENT WITH MEMBERS OF THE GENUS EHRLICHIA

Neorickettsia helminthoeca (tribe Ehrlichieae, family Rickettsiaceae) is the agent of salmon poisoning disease, which affects members of the family Canidae. This bacterium is unusual in that it is the only know n obligately intracellular bacterium that is transmitted via a helmint h vector, The nucleotide sequence of the N. helminthoeca 16S rRNA gene was determined and compared with the sequences of intracellular bacte ria belonging to the alpha subgroup of the Proteobacteria. The N. helm inthoeca sequence was most similar to the sequences of two Ehrlichia s pecies, Ehrlichia risticii and Ehrlichia sennetsu (levels of sequence similarity, >95%). All other members of the tribe Ehrlichieae, includi ng members of the other Ehrlichia species, and the related species Cow dria ruminantium and Anaplasma marginale, were only distantly related phylogenetically (levels of sequence similarity, 84 to 86%). Our resul ts corroborate the results of previous ultrastructural and Western blo t (immunoblot) comparisons of N. helminthoeca with other ehrlichial sp ecies. The genus Ehrlichia is phylogenetically incoherent and can be s eparated into three identifiable clusters of species. Each cluster is closely associated with a species classified in another non-Ehrlichia bacterial genus. The close relationships among N. helminthoeca, E. ris ticii, and E. sennetsu and the striking differences between these orga nisms and other members of the tribe Ehrlichieae suggest that in the f uture, these organisms should be treated as members of a new bacterial genus separate from the genus Ehrlichia.