Mechanism of acid trelance of oral streptococci

1.同时监测葡萄糖脉冲对洗涤过的链球菌细胞的NADH依赖性荧光和酸排泄。酸排泄开始于添加葡萄糖。同时，荧光迅速下降到最低水平并增加到平台水平。当酸排泄停止时，荧光从平台迅速增加到最大水平。结果表明，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.细菌细胞内pH（pHi）被认为高于细胞外pH（pHo），即使在不存在能量源如葡萄糖的情况下也是如此。在变形链球菌、血链球菌和唾液链球菌细胞悬液中加入乳酸，同时测定pHi和pHo。在不存在葡萄糖的情况下通过BCECF方法测量pHi。在pHo为7.5至5.0的范围内，pHi低于pHo，并在pHo为5至4的范围内保持恒定。pHi在pHo下从4降低至3.3。据推测，暴露于空气中的链球菌细胞中山梨醇代谢的速率可能受到氧气灭活丙酮酸甲酸裂解酶的限制。山梨糖醇产酸量的降低是由于山梨糖醇6-磷酸脱氢酶催化的反应速率降低所致，这是由于这些酶的底物短缺以及丙酮酸催化剂的减少。在山梨醇存在下，暴露于空气中的细胞可以开始代谢葡萄糖2分钟后，加入葡萄糖，作为葡萄糖转运的中间产物的积累。

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.