Mechanism of acid trelance of oral streptococci

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1. NADH-dependent fluorescence and acid excretion during glucose pulse to washed streptococcal cells were monitored simultaneously. Acid excretion started with addition of glucose. At the same time, the fluorescence dropped quickly to a minimum level and increased to a plateau level. When the acid excretion stopped, the fluorescence increased rapidly from the plateau to a maximum level. The results suggested that pyruvate metabolism started immediately after addition of glucose, then the rate of pyruvate metabolism became equal to the rate of glycolysis, and that the pyruvate metabolism stopped first when the acid excretion stopped.2. Bacterial intracellular pH (pHi) has been considered to be higher than extracellular pH (pHo) even if in the absence of energy source, such as glucose. The pHi and pHo were simultaneously measured when lactic acid was added to the S.mutans, S.sanguinis, S.salivarius cell suspensions. The pHi was measured by the BCECF method in the absence of glucose. The pHi was lower than the pHo at the range of pHo from 7.5 to 5.0 and was kept constant at pHo from 5 to 4. The pHi decreased at pHo from 4 to 3.3. It has been supposed that the rate of sorbitol metabolism in the air-exposed streptococcal cells could be limited by the inactivation of pyruvate formate-lyase by oxygen. The decreased acid production from sorbitol resulted from the reduced rates of the reactions catalyzed by sorbitol 6-phosphate dehydrogenase because of shortage of substrates for these enzymes in addition to the decrease in pyruvate catabolism. In the presence of sorbitol, the air-exposed cells could start to metabolize glucose 2 min after glucose was added, as the intermediates for the glucose transport were accumulated.