[Effects of oxygen on growth of Streptococcus mutans]

Effects of oxygen on growth of Streptococcus mutans Ingbritt (serotype c) were examined. The strain could be passaged under aerobic conditions at 37 degrees C in a peptone/yeast extract medium containing 0.2% glucose. Although the growth rate was slightly retarded under aerobic conditions (the doubling time, 60 min; 45 min under anaerobic conditions), the increment of cellular dry weight change was 35 g per mole of glucose under both conditions. To investigate such aerotolerance in growth as indicated by these results, oxygen metabolism was compared between aerobically and anaerobically grown cells. No difference was observed in the apparent rate of superoxide production or superoxide dismutation. However, aerobically grown cells showed a higher activity of hydrogen peroxide reduction with glucose as substrate and accumulated a lower level of hydrogen peroxide when incubated with glucose under aerobic conditions. In addition, hydrogen peroxide added to the medium was less inhibitory to anaerobic growth when aerobic cells were used as the inoculum. These results indicate that aerotolerance of S. mutans Ingbritt can be ascribed at least in part to its hydrogen peroxide reducing activity which is elevated under aerobic conditions.