PH REGULATION BY STREPTOCOCCUS-MUTANS

The intracellular pH (pH(i)) optimum for glycolysis in Streptococcus mutans Ingbritt was determined to be 7.0 by use of the ionophore gramicidin for manipulation of pH(i). Glycolytic activity decreased to zero as the pH(i) was lowered from 7.O to 5.0. In contrast, glycolysis had an extracellular pH(pH0)optimum of 6.O with a much broader profile. The relative insensitivity of glycolysis to the lowering of pH0 was attributed to the ability of S. mutans to maintain a transmembrane pH gradient (DELTA-pH, inside more alkaline) at low pH0. At a pH0 of 5.0, glycolyzing cells of S. mutans maintained a DELTA-pH of 1.37 +/- 0.09 units. The maintenance of this DELTA-pH was dependent on the concentration of potassium ions in the extracellular medium. Potassium was rapidly taken up by glycolyzing cells of S. mutans at a rate of 70 nmol/mg dry weight/min. This uptake was dependent on the presence of both ATP and a proton motive-force (DELTA-p). The addition of N-N'-dicyclohexylcarbodiimide (DCCD) to glycolyzing cells of S. mutans caused a partial collapse of the DELTA-pH. Growth of S. mutans at pH0 5.5 in continuous culture resulted in the maintenance of a DELTA-pH larger than that produced by cells grown at pH0 7.0. These results suggest the presence of a proton-translocating F1F0-ATPase in S. mutans whose activity is regulated by the intracellular pH and transmembrane electrical potential (DELTA-psi). The production of an artificial DELTA-p of 124 mV across the cell membrane of S. mutans did not result in proton movement through the F1F0-ATPase coupled to ATP synthesis. This suggests that ATP synthesis is not driven by the low DELTA-p values obtained under physiological conditions in S. mutans. Therefore, the major role of the F1F0-ATPase is likely to be in pH(i) regulation. The extracellular concentration of sodium had no effect on DELTA-pH maintenance in glycolyzing cells, whereas the addition of 500-mu-mol/L fluoride caused a significant fall in pH(i). S. mutans was able to grow in the absence of a DELTA-p, indicating that the transmembrane circulation of protons is not obligatory for growth of this organism.