Three Kinds of Mutans Streptococci from Pig Oral Cavity

Objectives: To investigate the phylogenetic affinities of Streptococcus mutans , a group of Gram-positive cocci from oral flora of pig was focused and characterized. Methods: The bacteria were analyzed biochemical using the Rapid ID 32 Strep Microsystems and serologically using anti-sera of mutans streptococci, were subjected to DNA-DNA hybridization with other members of the mutans streptococci and had their 16S rRNA gene analyzed. Results: Three kinds of typical S. mutans-like colonies from each 39 pig were isolated on Mitis Salivarius Agar from oral cavities. The strains formed small, raised, irregular in margin and adherent colonies. These characteristics of the isolates resembled those of mutans streptococci. One strain was grouped in serotype c of mutans streptococci and identified Streptococcus ferus by homology of 16S rRNA gene. However, the other two strains were not classified in the given serotype groups. To determine the phylogenetic affinities of these species, its almost complete 16S rRNA gene was sequenced and subjected to a comparative analysis. Highest sequence relatedness was shown with Streptococcus criceti. DNA-DNA hybridization showed that the percent homology of clinical isolates and S. criceti was 50~75%. Also these two clinical isolates were 60~75% homology by DNA-DNA hybridization each other. These results indicated that two clinical isolates were different species. Based on phylogenetic and phenotypic evidence, one species was S. ferus and two new species were proposed Streptococcus suisoris and Streptococcus dentsuis. Conclusion: This study shows that there are at least 3 kinds of mutans streptococci in the oral cavity of pigs. This study was supported in part by a Grant-in-Aid for Nihon University Individual Research Grant for 2005.