Studies with fluoride-sensitive and fluoride-resistant strains of Streptococcus salivarius. II. Fluoride inhibition of glucose metabolism

Abstract

Low concentrations of sodium fluoride inhibited use of exogenous glucose by washed cells of the homofermentative oral microbe Streptococcus salivarius whether added before or after the substrate. Complete inhibition of exogenous glucose metabolism was achieved by NaF concentrations above 0.96 mM. Inhibition of glucose uptake caused an immediate reduction in the lactic acid formed as a result of metabolism, although the percentage conversion of glucose to lactic acid was not altered by fluoride at levels below 0.24 mM. Above this concentration, short periods of non-stoichiometric lactate formation were observed but continued incubation in the presence of fluoride always resulted in the production of normal quantities of lactic acid. Fluoride inhibition of glucose metabolism was pH-dependent with complete inhibition by 0.12 mM NaF occurring at pH values below 6.0. Sufficient fluoride was absorbed by cells at fluoride concentrations above 1.2 mM in a 30-minute period to reduce significantly subsequent glycolytic activity by the cells in medium free of the inhibitor.Mutants resistant to 2.4 mM NaF were shown to degrade glucose in the presence of 2.4 and 4.8 mM NaF at rates 74–46% of that of the control cells grown in the absence of this inhibitor. Growth of these resistant cells for 300 generations in medium free of fluoride stimulated the rate of glucose metabolism in the presence of 2.4 mM NaF, but exposure to 4.8 mM NaF reduced the rate of activity to 30% of that of the same cells incubated at 2.4 mM NaF. Comparison of the fluoride sensitivity of exogenous glucose metabolism and of endogenous polyglucose degradation have indicated that the prime site of fluoride action in these cells was at some point before glucose-6-P formation, possibly involved with the glucose transport process, the phosphorylation of glucose by hexokinase or an associated reaction. This conclusion was confirmed in recent experiments which demonstrated that the addition of low concentrations of fluoride to cells incubated with glucose resulted in an immediate reduction in the intracellular level of glucose-6-P.