Reclassification of Treponema hyodysenteriae and Treponema innocens in a New Genus, Serpula gen. nov., as Serpula hyodysenteriae comb. nov. and Serpula innocens comb. nov.
The intestinal anaerobic spirochetes Treponema hyodysenteriae B78T (T = type strain), B204, B169, and A-1, Treponema innocens B256T and 4/71, Treponema succinifaciens 6091T, and Treponema bryantii RUS-1T 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 T. hyodysenteriae B78T and B204 had 93% sequence homology with each other and approximately 40% sequence homology with T. innocens B256T and 4/71. Both T. hyodysenteriae B78T and T. innocens B256T exhibited negligible levels of DNA homology (96%) in electrophoretic profiles among T. hyodysenteriae strains, moderate levels of similarity (43 to 49%) between T. hyodysenteriae and T. innocens, and no detectable similarity between the profiles of either T. hyodysenteriae or T. innocens and those of T. succinifaciens, T. bryantii, and Escherichia coli. Restriction endonuclease analysis of DNA was not useful in assessing genetic relationships since there was heterogeneity even between strains of T. hyodysenteriae. 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 T. hyodysenteriae B78T, B204, and A-1 were nearly identical (99.8 to 99.9% base sequence similarity). T. innocens B256T and 4/71 were closely related to the T. hyodysenteriae strains (99.4 and 99.0% similarity). Strains of T. hyodysenteriae and T. innocens exhibited low levels of 16S rRNA similarity (average, 76.5%) with T. pallidum, Borrelia burgdorferi, and various other spirochetes. The results of our investigations indicate that T. hyodysenteriae and T. innocens are distinct but related species of spirochetes. T. hyodysenteriae and T. innocens are only distantly related to T. pallidum, the type species of the genus Treponema, and to other spirochetes. Consequently, we propose that the species T. hyodysenteriae and T. innocens be transferred to a new genus, Serpula, gen. nov.