Effect of low levels of fluoride on proton excretion and intracellular pH in glycolysing streptococcal cells under strictly anaerobic conditions

The effect of low levels of fluoride on intracellular acid production and proton excretion in Streptococcus mutans NCTC 10449 at different growth and extracellular pH (pH(o)) levels was monitored under strictly anaerobic conditions. The sensitivity of S. mutans to fluoride increased as pH, decreased. Cells grown under acidic (pH 6.0 and 5.5) conditions were more resistant to fluoride than cells grown at a neutral pH. Under acidic extracellular conditions (pH(o)=5.0), 0.025 mM fluoride inhibited proton excretion by approximately 50% in cells grown at pH 7.0. Slightly higher levels of fluoride (0.05-0.07 mM) were required for 50% or more inhibition in cells exposed to alkaline extracellular conditions or cells grown at acidic pH. Such levels of fluoride are about 10-20 times lower than that reported previously. Therefore, it is possible that as pH falls during initial bacterial glycolysis, sufficient amounts of anionic fluoride may be released, from its bound form in plaque, to cause significant inhibition of net proton movement out of the bacterial cell during further glycolysis, especially under the anaerobic environment of the deep layers of plaque. In addition to proton excretion, fluoride was also found to inhibit inh acellular acid production.