Comparison of the effects of fluoride and the ionophore nigericin on acid production by Streptococcus mutans and the resultant in vitro enamel demineralization.

In an in vitro demineralization experiment with a plaque of S. mutans C180-2 over enamel, the effect of 0.5 mmol fluoride/L was compared with that of 10 nmol nigericin/L. Since the effects of both substances on the acid production in a dense cell suspension were of the same magnitude, and since nigericin did not affect the enamel solubility, the present demineralization experiment may differentiate between the effect of fluoride on bacterial metabolism alone and the combined effect of fluoride on both the bacterial metabolism and enamel solubility. In our experiments, the effect on bacterial metabolism accounted for 75% of the caries-inhibiting effect of fluoride. Microradiographic data showed subsurface lesions in the control and nigericin group and a slight surface-softening in the fluoride group. In addition, the effects of fluoride and nigericin on the cellular internal pH during glycolysis were studied. Analysis of these data suggested that glycolysis was inhibited when the intracellular pH reached about pH 5.2, which supports the view that acidification of the cell cytoplasm is a mechanism by which fluoride inhibits glycolysis.