Streptococcal species belonging to the salivarius group are shown in orange (S. salivarius), blue (S. vestibularis) or green (S. thermophilus). Other streptococcal species shown in black were outgroups. Branch lengths are drawn to scale. Discussion When we began our study, we expected that the S. salivarius and S. vestibularis species would be more closely related to each other given their level of physiological
resemblance and that the S. vestibularis/S. thermophilus sister-relationship inferred in previous phylogenetic studies [2, 14] would not be robustly supported. Obviously, this was not the case. Our results were in complete agreement with earlier neighbor-joining phylogenies based on partial 16S rRNA-encoding Selleck PRI-724 and sodA gene sequences [2, 14] and corroborated the S. vestibularis/S. thermophilus sister-relationship. This sister-relationship was not dependent on the method of phylogenetic reconstruction and was strongly supported by both our ML and MP analyses. Furthermore, while the 16S-rRNA-encoding check details and secY
gene sequences were unable to discriminate SRT1720 solubility dmso between the S. vestibularis/S. thermophilus and the alternate S. vestibularis/S. salivarius and S. salivarius/S. thermophilus sister-relationships, we observed no serious incongruities between the topologies inferred from these molecular markers and those inferred from the recA and secA gene sequences. The S. vestibularis/S. thermophilus sister-relationship inferred from our phylogenetic analyses is not necessarily incompatible with the observation that S. vestibularis share more phenotypic similarities with S. salivarius than with S. thermophilus. Following speciation from a putative common ancestor physiologically similar to S. salivarius, PFKL the two newly formed species could have evolved differently, with S. vestibularis and S. thermophilus independently retaining and discarding a number of ancestral features. Many of the phenotypic losses observed in the S. thermophilus species could have been induced
by its adaptation to its new ecosystem, i.e., the bovine mammary mucosa. In particular, because this species has access to a wealth of nutrients within bovine milk, polyvalence for sugar metabolism-related genes might not be as important for this species as for its relatives inhabiting the human oral mucosa . Further losses could have been caused by additional selective pressure applied on S. thermophilus commercial strains ( and references therein) that are used in the manufacture of various dairy products. The relationships inferred among the three salivarius streptococci raise interesting questions regarding their establishment in their respective ecosystems. Because the S. salivarius/S. vestibularis sister-relationship is not supported by phylogenetic analyses, the colonization of the human oral cavity by an ancestor of S. thermophilus present in bovine milk, which would have then speciated over time into S.