Divergence in the level of DNA hybridization and formation of sibling species in the lactic acid bacteria Streptococcus thermophilus

Previously, five distinct groups with 80-90% intragroup DNA homology values were revealed among 19 lactic acid-producing bacterial strains. The study of 39 new strains of thermophilic streptococci in the present work allowed us to reveal the sixth DNA homology group. The nine strains of this group are similar, at 55-70% DNA homology levels, to the type strain S. thermophilus ATCC 19258. Group VI showed a low level of DNA-DNA reassociation (20-30%) with the DNA homology groups I, II, III, and V. The intergroup DNA-DNA reassociation values determined from DNA renaturation rates varied from 20 to 50%. These data were interpreted as indicative of the existence of at least four sibling species among the thermophilic streptococci studied.