Characterization of a mutant of serotype g Streptococcus mutans strain 6715 lacking dextran-induced agglutination.

A spontaneous mutant of Streptococcus mutans 6715 (serotype g) defective in dextran-induced agglutination ability was isolated. The wild type strain and its mutant were termed as 6715-DP and 6715-DN, respectively. Both strains possessed serotype g antigen, and exerted similar sugar fermentation patterns. Strain 6715-DP was rapidly and strongly agglutinated upon addition of high molecular weight dextran, whereas the mutant strain 6715-DN was not. [14C]Dextran prepared from Leuconostoc mesenteroides dextransucrase and [14C]sucrose bound to fresh or lyophilized 6715-DP cells, but not to the mutant 6715-DN cells. However, both strains adhered to a glass surface in the presence of sucrose. Furthermore, heat-treated (100 degrees C, 10 min) cells of both strains bound cell-free glucosyltransferase, although dextran agglutination ability of strain 6715-DP was destroyed by this treatment, indicating that receptors for dextran and glucosyltransferase were different entities. Furthermore, serotype c, e, and f strains S. mutans did not agglutinate upon addition of dextran, nor did they bind [14C]dextran. However, all these strains and both 6715-DP and 6715-DN strains induced marked dental caries in SPF rats. It is concluded that dextran-induced agglutination ability is not a necessary condition for S. mutans to induce dental caries.