Effects of environmental fluctuations on sugar metabolisms of plaque bacteria.

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When oral streptococci was cultured at low pH in continous culture under strictly anaerobic conditions, more lactate was produced with a concomitant decrease in formate and acetate, compared with at neutral pH. Two mechanisms seemed to be mainly involved in the increased production of lactate at acidic pH in the chemostat. Firstly, lactate dehydrogenase (LDH) had lower optimum pH than that of pyruvate formate-lyase (PFL). Secondly, the amount of LDH was markedly increased due to the acidification in the cells. In contrast to other streptococci S. mutans could produce more acid even at pH 5.1-5.5 because of acid tolerance of ATPase which excrete proton from the cells. These properties were considered to render S. mutans higher cariogenicity.It is essential for plaque microorganisms to keep intracellular NADH/NAD balance under anaerobic conditions. Oral streptococci employed the PFL system to metabolize sorbitol anaerobically. Lactobacilli could regenerate NAD to keep the balance via the conversion of acetate into ethanol and metabolize sorbitol in the presence of availabe acetate, even when PFL was irreversibly inactivated. Actinomyces could regenerate NAD via succinate pathway when bicarbonate was available, and increased the rate of acid production three- to four-fold. These effects were not observed in streptococci.Various environmental factors affect sugar metabolisms of several plaque microorganisms, but these effects are quite different depend on bacterial species.