16S rRNA gene sequence of Neorickettsia helminthoeca and its phylogenetic 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 known obligately intracellular bacterium that is transmitted via a helminth vector. The nucleotide sequence of the N. helminthoeca 16S rRNA gene was determined and compared with the sequences of intracellular bacteria belonging to the alpha subgroup of the Proteobacteria. The N. helminthoeca sequence was most similar to the sequences of two Ehrlichia species, Ehrlichia risticii and Ehrlichia sennetsu (levels of sequence similarity, > 95%). All other members of the tribe Ehrlichieae, including members of the other Ehrlichia species, and the related species Cowdria ruminantium and Anaplasma marginale, were only distantly related phylogenetically (levels of sequence similarity, 84 to 86%). Our results corroborate the results of previous ultrastructural and Western blot (immunoblot) comparisons of N. helminthoeca with other ehrlichial species. The genus Ehrlichia is phylogenetically incoherent and can be separated 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. risticii, and E. sennetsu and the striking differences between these organisms and other members of the tribe Ehrlichieae suggest that in the future, these organisms should be treated as members of a new bacterial genus separate from the genus Ehrlichia.