To the Editor: Five species of tick-associated rickettsiae have been identified in China; of these, three are human pathogens and two are of unknown pathogenicity (1). In 1990, one isolate, BJ-90, was first obtained from a Dermacentor sinicus tick, a newly recognized vector collected in a Beijing suburb, an atypical location for Rickettsia sibirica (2). Several taxonomic studies of the phenotype, antigenicity, and genotype of BJ-90 have been performed, with inconsistent results (2-6). Recently, phylogenetic analysis based on several gene comparisons has enabled the phylogenetic classification of this rickettsial species (7-11). To confirm the phylogenetic relationships between the BJ-90 strain and other rickettsiae, the 16S rRNA, gltA, and OmpA encoding genes were amplified and sequenced. Phylogenetic relationships between the BJ-90 strain and other rickettsia in the GenBank database were inferred by the parsimony and neighbor-joining methods (9). Bootstrap analyses were used to assess the reliability of the phylogenetic analysis.

Both methods showed a high degree of similarity between BJ-90, R. sibirica and R. mongolotimonae, which were grouped in the same cluster in three inferred dendrograms. The data from the 16S rRNA and gltA sequences showed low statistical significance in the cluster (bootstrap values for the nodes 50% and 33%, respectively). However, data from the rompA gene sequence showed highly significant similarity in the cluster (bootstrap value 100%), confirming the reliability of the phylogenetic analysis. The results of this phylogenetic analysis are consistent with those of previous phenotypic, genotypic, and phylogenetic analyses (2,3,5-11), as well as taxonomy derived from direct antigenic comparison of the species (4). The sequences of 16S rRNA, gltA, and OmpA have been assigned the following GenBank accession numbers: AF178036 for 16S rRNA, AF178035 for gltA, AF179365 for the 611-bp sequence of ompA, and AF179367 for the 3174-bp sequence of ompA. According to previous genotypic and antigenic studies and our phylogenetic analysis, in which the BJ-90 strain is closer to R. sibirica than R. mongolotimonae in the dendrogram inferred from comparison of the ompA encoding gene sequences, the BJ-90 strain should be considered a variant of R. sibirica.

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