The ixodid tick is the most important vector of Lyme disease in Japan. Most spirochete isolates obtained from I. persulcatus ticks have been classified as sensu lato because of their genetic, biological, and immunological characteristics. However, we found that a small number of isolates obtained from contained a smaller 38-kDa endoflagellar protein and single 23S-5S rRNA gene unit. Representative isolate HT31 (T = type strain) had the same 23S rRNA gene physical map as The DNA base composition of strain HT31 was 28.6 mol% G+C. DNA-DNA hybridization experiments revealed that strain HT31 exhibited moderate levels of DNA relatedness (24 to 51%) with , and However, the levels of DNA reassociation with the previously described Lyme disease borreliae (, and ) were only 8 to 13%. None of the previously described species examined exhibited a high level of DNA relatedness with strain HT31. In addition, the 16S rRNA gene sequence (length, 1,368 nucleotides) of strain HT31 was determined and aligned with the 16S rRNA sequences of other species. Distance matrix analyses were performed, and a phylogenetic tree was constructed. The results showed that isolate HT31 is only distantly related to both previously described Lyme disease borreliae and relapsing fever borreliae. Thus, the spirochetes isolated from and closely related isolates should be classified as members of a new species. We propose the name sp. nov. for this spirochete; strain HT31 is the type strain.


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