Suppression of Streptococcus sobrinus 6715 (g) in plaques by Streptococcus mutans 32K (c)

The dental plaque of 96 healthy donors was screened for the production of such antibacterial substances as mutalipocin and bacteriocin and 192 strains of mutans streptococci isolated: 28 produced mutalipocin and 22 produced bacteriocin. Mutalipocin produced by these 28 S. mutans strains possessed similar biochemical and biological characteristics of well-characterized mutalipocin-producing strain S. mutans 32K (serotype c). When equal amounts of S. mutans 32K and S. sobrinus 6715 (g) were cultured together, cells of S. sobrinus 6715 were completely killed in 18 h. In addition, S. mutans 32K inhibited in vitro plaque formation by S. sobrinus 6715, and S. mutans 32K also eliminated in vitro plaque preformed by S. sobrinus 6715. In rat experiments, S. mutans 32K could pre-emptively colonize in plaque preformed by S. sobrinus 6715. On the other hand, S. sobrinus 6715 could not colonize in plaque preformed by S. mutans 32K. The results indicate that S. mutans serotype c which produces antibacterial substances is able to invade dental plaque and replace the other mutans streptococci. This investigation offers one of the possible explanation why S. mutans serotype c is a predominant species among mutans streptococci in human plaque.