Inhibition of Actinomyces viscosus by bacteriocin-producing strains of Streptococcus mutans in the dental plaque of gnotobiotic rats

Gnotobiotic rats were infected with both Actinomyces viscosus Ny 1 and various strains of Streptococcus mutans. The simultaneous inoculation of Strep. mutans. OMZ 1766, a strain devoid of bacteriocin activity against A. viscosus Ny 1 in vitro, did not prevent the establishment of strain Ny 1 in the plaque. In contrast, Strep. mutans C 67-1, which is bacteriocinogenic against A. viscosus Ny 1, prevented the establishment of strain Ny 1 when it was introduced up to two days before the Strep. mutans C 67-1. A. viscosus Ny 1 could only be established if it was inoculated 13 days before infection with Strep. mutans C 67-1. Electron microscopic examination of these plaques supported this antagonistic relationship. The same phenomenon was observed with Strep. mutans T2, a strain that also shows in-vitro bacteriocin activity against A. viscosus Ny 1. In contrast, strain Ny 1 was successfully established in the presence of mutagen-induced non-bacteriocinogenic isolates of Strep. mutans C 67-1 and T2. These findings suggest that although bacteriocins may be important in the regulation of the flora in plaque, at least on a micro-ecological scale, they do not necessarily completely eliminate the sensitive species.