Dissipation of the proton motive force in oral streptococci by fluoride

Strains of oral streptococci maintain an intracellular pH (pHi) that is more alkaline than the external pH (pHo). The delta pH (pHi-pHo) was about 0.6 in neutral media and about 0.9 in media of pH 5.5. Addition of 10 micrograms of F- per ml at pH 7.0 reduced the delta pH in Streptococcus salivarius to 0.17. The reduction of delta pH in S. sanguis H7PR3 was less pronounced, whereas the delta pH in S. mutans IB 1600 was unaffected. The F- -resistant mutant of S. salivarius, strain Flr103, maintained a delta pH of 0.51 with 100 micrograms of F- per ml. Addition of F- to cells in media below pH 6.0 led to a reduction of delta pH in all strains. The anion had no effect on the transmembrane electrical gradient of either mutant or parental cells of S. salivarius at pH 7.0. The principal effect of F- addition at neutral pH, therefore, was on the delta pH component of the proton motive force. At pH 5.5, 100 micrograms of F- per ml reduced the transmembrane electrical gradient from 71 to 40 mV in the parent and from 80 to 42 mV in the mutant. We propose that the greater sensitivity of cells to F- at lower medium pH stems from the rapid dissipation of delta pH by the anion. Thus, pH equilibration in media of low pH would lead to a greater reduction of metabolic activity than when it occurs in media at neutral pH. The data also suggest that the growth of streptococci, with or without added F-, is limited when the intracellular pH falls below about 5.7.