Mechanism of acid trelance of oral streptococci

1.同时监测了洗涤后的链球菌细胞在葡萄糖脉冲作用下的NADH依赖的荧光和排酸情况。酸的排泄始于添加葡萄糖。与此同时，荧光迅速下降到最低水平，并增加到平台期。当排酸停止时，荧光从平台迅速增加到最大值。结果表明，在添加葡萄糖后，丙酮酸代谢立即开始，然后丙酮酸代谢速率与糖酵解速率相等，当排酸停止时丙酮酸代谢首先停止。细菌的胞内pH(Phi)一直被认为高于胞外pH(Pho)，即使在没有葡萄糖等能源的情况下也是如此。同时测定乳酸对变形链球菌、血链球菌、唾液链球菌细胞悬液的phi和pho的影响。在无糖条件下，用BCECF法测定pHi。Phi在pho为7.5~5.0的范围内低于pho，在pho为5~4时保持恒定，在pho为4~3.3时有所下降。据推测，暴露在空气中的链球菌细胞的山梨醇代谢速率可能受到氧气对丙酮酸甲酸裂解酶失活的限制。山梨醇的产酸减少是由于山梨醇6-磷酸脱氢酶催化的反应速率降低，这是因为这些酶的底物不足，以及丙酮酸分解代谢的减少。在山梨醇的存在下，暴露于空气中的细胞在添加葡萄糖后2min即可开始代谢葡萄糖，因为葡萄糖转运的中间产物已经积累。

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.