The intestinal anaerobic spirochetes B78 (T = type strain), B204, B169, and A-1, B256 and 4/71, 6091, and RUS-1 were compared by performing DNA-DNA reassociation experiments, sodium dodecyl sulfate-polyacrylamide gel electrophoresis of cell proteins, restriction endonuclease analysis of DNA, and 16S rRNA sequence analysis. DNA-DNA relative reassociation experiments in which the S1 nuclease method was used showed that B78 and B204 had 93% sequence homology with each other and approximately 40% sequence homology with B256 and 4/71. Both B78 and B256 exhibited negligible levels of DNA homology (<5%) with 6091. The results of comparisons of protein electrophoretic profiles corroborated the DNA-DNA reassociation results. We found high levels of similarity (>96%) in electrophoretic profiles among strains, moderate levels of similarity (43 to 49%) between and , and no detectable similarity between the profiles of either or and those of , and . Restriction endonuclease analysis of DNA was not useful in assessing genetic relationships since there was heterogeneity even between strains of . Partial 16S rRNA sequences of the intestinal spirochetes were determined by using a modified Sanger method and were compared in order to evaluate the phylogenetic relationships among these and other spirochetes. The 16S rRNA sequences of B78, B204, and A-1 were nearly identical (99.8 to 99.9% base sequence similarity). B256 and 4/71 were closely related to the strains (99.4 and 99.0% similarity). Strains of and exhibited low levels of 16S rRNA similarity (average, 76.5%) with , and various other spirochetes. The results of our investigations indicate that and are distinct but related species of spirochetes. and are only distantly related to , the type species of the genus , and to other spirochetes. Consequently, we propose that the species and be transferred to a new genus, , gen. nov.


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