Adaptation by Streptococcus mutans to acid tolerance

Our previous continuous culture studies with strains of Streptococcus mutans have indicated that the organism has the capacity of adapt to growth in acidic environments. This study was undertaken to examine this question in more detail. S. mutans Ingbritt and the phosphotransferase system (PTS)‐defective mutant, S. mutans DR0001/6, were grown in continuous culture at pH 7.5 and 5.5 or 5.1, and the pH optimum for glucose uptake and glycolysis and the capacity of the cells to generate pH gradients were determined over the pH range 4.5 to 8.0 with steady state, washed cells. In addition, the proton permeability of the cells was measured over the pH range by an acid pulse technique. The results indicate that the pH optimum for glucose uptake by S. mutans Ingbritt grown at pH 7.5 was 7.5 and this optimum shifted to 7.0 and 6.0 for cells grown at pH 5.5 and 5.1, whereas with the S. mutans DR0001/6, the optimum shifted from 7.5 to 7.0 for the pH 5.5 cells. A similar shift in the pH optimum for glycolysis was observed for the 2 strains, and this was particularly pronounced for cells incubated with glucose in the presence of gramicidin to dissipate proton gradients. The capacity of the cells to generate pH gradients was related to their metabolic activity, and although larger gradients were not formed by the pH 5.5 cells, these cells were nevertheless capable of maintaining gradients at a lower pH; S. mutans DR0001/ 6 generated 2‐fold larger pH gradients at pH 5.5 than S. mutans Ingbritt. In addition, activity for the membrane‐associated proton‐extruding ATPase in S. mutans Ingbritt increased 4‐fold as the pH of growth was decreased from 8.0 to 5.0. The most striking difference between cells grown at pH 7.5 and 5.5 was observed in the proton permeability profiles for the 2 organisms. The pH for minimum proton permeability for the pH 7.5‐grown cells of S. mutans Ingbritt Was 6.4, whereas that for pH 5.5 cells was 5.0; the corresponding values for S. mutans DR0001/6 were pH 5.6 and 4.6 for the pH 7.5 and 5.5 cells, respectively. This indicates that the cells compensate for the lower external pH by decreasing the permeability of protons into the cell at the lower pH values. Collectively, these results indicate that prolonged growth of S. mutans in an acidic environment, such as might be found in fissures or carious lesions, results in significant changes in cell physiology that confer increased acidurance.