INHIBITION OF ACID PRODUCTION IN STREPTOCOCCUS-MUTANS R9 - INHIBITIONCONSTANTS AND REVERSIBILITY

End-product inhibition of acid production in Streptococcus mutans R9 w as investigated by measuring effects of varying concentrations of H+ a nd of undissociated lactic or acetic acids on acid production rates in the pH stat. H+ caused purely uncompetitive inhibition (inhibition co nstant K-iu 0.018 mmol l(-1)). Lactic acid caused mixed inhibition wit h inhibition constants of K-iu (uncompetitive) 4.45 mmol l(-1) and K-i c (competitive) 2.28 mmol l(-1), as did acetic acid with inhibition co nstants of K-iu 4.24 mmol l(-1) and K-ic 4.55 mmol l(-1) Reversibility of inhibition by H+ showed only a statistically significant reduction only at pH < 4.5. Reversibility of inhibition by lactic and acetic ac ids decreased with increasing inhibitor concentration. In all cases, r eversibility correlated with the extent to which viability was retaine d, 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 organis ms to produce acid in subsequent exposures to carbohydrate may be redu ced.