In vitro effects of low concentrations of penicillin and sulfadiazine on Streptococcus mutans

Investigations were conducted to determine the in vitro effects of low levels of penicillin and sulfadiazine on the growth, plaque formation, and agglutination of Streptococcus mutans and on the synthesis and activity of enzymes synthesizing extracellular polymers. The concentrations tested were equivalent to those expected in the saliva of subjects receiving oral therapy with the agents. Penicillin at 0.5 ng/ml and sulfadiazine at 1 mug/ml substantially inhibited in vitro plaque formation. At these concentrations, sulfadiazine but not penicillin also inhibited growth of the organism. Neither antimicrobial agent affected the agglutination of S. mutans with dextran or the synthesis or activity of enzymes synthesizing extracellular polymers. The effect of sulfadiazine on plaque formation was attributed, at least in part, to the inhibitory action of that agent on S. mutans growth.