Inhibition of acid production in Streptococcus mutans R9: inhibition constants and reversibility.

End-product inhibition of acid production in Streptococcus mutans R9 was investigated by measuring effects of varying concentrations of H+ and of undissociated lactic or acetic acids on acid production rates in the pH stat. H+ caused purely uncompetitive inhibition (inhibition constant Kiu 0.018 mmol 1(-1). Lactic acid caused mixed inhibition with inhibition constants of Kiu 4.24 mmol 1(-1) and Kic 4.55 mmol 1(-1). Reversibility of inhibition by H+ showed only a statistically significant reduction only at pH < 4.5. Reversibility of inhibition by lactic and acetic acids decreased with increasing inhibitor concentration. In all cases, reversibility correlated with the extent to which viability was retained, suggesting that loss of reversibility was due to cell death. These results suggest that, after a low-pH episode in dental plaque, caused by fermentation of dietary carbohydrate, the ability of plaque organisms to produce acid in subsequent exposures to carbohydrate may be reduced.