Evolutionary relationships among salivarius streptococci as inferred from multilocus phylogenies based on 16S rRNA-encoding, recA, secA, and secY gene sequences

The maximum likelihood (ML) and maximum parsimony (MP) phylogenetic inferences derived from the secA and recA gene sequences provided strong support for the S. vestibularis/S. thermophilus sister-relationship, whereas 16S rRNA-encoding and secY-based analyses could not discriminate between alternate topologies. Phylogenetic analyses derived from the concatenation of these sequences unambiguously supported the close affiliation of S. vestibularis and S. thermophilus.Our results corroborated the sister-relationship between S. vestibularis and S. thermophilus and the concomitant early divergence of S. salivarius at the base of the salivarius lineage.The Streptococcus genus comprises ninety-two recognized species that are present in a wide variety of habitats [1]. In humans and animals, a number of streptococcal species are important pathogens (e.g., S. pneumoniae, S. pyogenes, S. suis, and S. mutans), while others are members of mutualistic microflora (e.g., S. oralis, S. downei, S. dentirousetti, and S. salivarius). The species of the Streptococcus genus have been divided into six groups (anginosus, bovis, mitis, mutans, pyogenic, and salivarius) based on 16S rDNA phylogenetic inferences [2]. According to these authors, the salivarius group is composed of three species: (1) S. salivarius, a pioneer colonizer of the human oral mucosa that is isolated mainly from the dorsum of the tongue, the cheeks, and the palate [3], (2) S. vestibularis, a mutualistic bacterium that is present on the vestibulum of the human oral mucosa [4], and (3) S. thermophilus, a thermophilic species [5] that is part of starter cultures used in the production of yogurt and Swiss- or Italian-type cooked cheeses. Unlike S. salivarius and S. vestibularis, S. thermophilus is not a natural inhabitant of the human oral mucosa and is commonly found on the mammary mucosa of bovines, its natural ecosystem, as inferred from its presence and that of thermophilus-specific bacteriophages in raw milk isolates