Fluoride acts to reduce acid tolerances of plaque bacteria by upsetting normal proton currents across cell membranes. Streptococcus mutans was found to be unusually sensitive to fluoride, in part because its F₁F₀, proton-translocating ATPase is directly inhibited by fluoride at plaque levels. Thus, not only does fluoride serve in the HF form to bring extruded protons back into the cell, but it also reduces the capacity of the cell to extrude protons. Reductions in acid tolerance caused by fluoride would be expected to result in concomitant reductions in cariogenic potential.