A spirochete which infects short-tailed shrews () and white-footed mice () has been shown previously to be ultrastructurally and serologically distinct from other spirochetes. Two of the original isolates from Connecticut and Minnesota and 16 new isolates obtained from shrews captured in Minnesota were characterized phenotypically and genetically in this study. A comparative analysis of the 16S rRNA sequences of two shrew isolates and one mouse isolate and the 16S rRNA sequences of 16 other spirochetes and revealed that these organisms exhibited low levels of similarity (range of similarity values, 73.9 to 77.8%; average level of similarity, 74.7%). The shrew and mouse isolates which we examined formed a deeply branching subgroup that was clearly distinct from the other genera of spirochetes examined. These and other results indicated that the new spirochetes represent a unique taxon in the order . Accordingly, we propose that they should be classified as members of a new genus, . The three strains of which we examined had 16S rRNA sequences that were nearly identical. We also compared these isolates by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, fatty acid and enzyme analyses, restriction endonuclease analysis, and Southern hybridization and found that the levels of genetic and phenotypic homogeneity among the strains were very high. We concluded that the isolates which we examined were members of a single species, for which we propose the name . The type strain of is CT11616 (= ATCC 43811). The guanine-plus-cytosine content of the DNA of this species was determined to be 34 to 36 mol% by the thermal denaturation method.


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