Interactions between and within Streptococcus mutans and Streptococcus sobrinus isolated from humans harboring both species.

The prevalence of Streptococcus mutans and Streptococcus sobrinus was examined in plaque samples from small discrete areas of the buccal tooth surfaces of seven subjects. Strains of S. mutans and S. sobrinus were isolated and tested for bacteriocin-mediated interactions between and within the two species, using the stab inoculation technique. S. mutans and S. sobrinus did not colonize each tooth surface uniformly and, in plaque from small discrete sites, S. mutans and S. sobrinus were either undetected or present in different interspecies proportions. Within the same subject, there were no bacteriocin-mediated interactions between strains of the same mutans species and no difference in bacteriocin activity was found between the strains of S. mutans and S. sobrinus from different sites. When bacteriocin interactions were tested between isolated strains from all seven subjects a somewhat higher inhibition ability was found for producer strains isolated from plaque compared with those isolated from saliva. S. mutans appeared to be more bacteriocinogenic than S. sobrinus. Replacing the glucose in the medium with sucrose enhanced the bacteriocin activity of S. mutans towards other S. mutans strains but reduced the inhibitory interaction towards strains of S. sobrinus.