Dissipation of the proton motive force in oral streptococci by fluoride

Abstract

Strains of oral streptococci maintain an intracellular pH (pH(i)) that is more alkaline than the external pH (pH(o)). The ΔpH (pH(i)-pH(o)) was about 0.6 in neutral media and about 0.9 in media of pH 5.5. Addition of 10 μg of F- per ml at pH 7.0 reduced the ΔpH in Streptococcus salivarius to 0.17. The reduction of ΔpH in S. sanguis H7PR3 was less pronounced, whereas the ΔpH in S. mutans IB 1600 was unaffected. The F--resistant mutant of S. salivarius, strain Flr103, maintained a ΔpH of 0.51 with 100μg of F- per ml. Addition of F- to cells in media below pH 6.0 led to a reduction of ΔpH 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 ΔpH component of the proton motive force. At pH 5.5, 100 μg 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 ΔpH 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.