Acid production by streptococci growing at low pH in a chemostat under anaerobic conditions

Streptococcus mutans and other oral streptococci were grown in continuous culture under strictly anaerobic conditions. When the cultural pH was kept at 7.0, the main acid products were formate and acetate, as reported previously. However, more lactate was produced at pH values of 5.5 or 6.0, with a concomitant decrease in formate and acetate production. This change in fermentation products could partly be ascribed to a change in intracellular pH and difference in the pH optima between pyruvate formate-lyase (PFL) and lactate dehydrogenase (LDH). At extracellular pH values of 7.0 and 5.5, the intracellular pH values of S. mutans NCIB 11723 were 7.5 and 6.6, respectively. The pH optima of PFL and LDH were 7.8 and 5.5-6.3, respectively. The cells had also a larger amount of LDH during growth at pH 5.5 than at pH 7.0.