EFFECT OF SODIUM FLUORIDE ON ORAL LACTOBACILLI ISOLATED FROM THE RAT

¹ National Institute of Dental Research, National Institutes of Health, Bethesda 14, Md.

When varying concentrations of sodium fluoride were used to inhibit the growth of 20 strains each of homofermentative and heterofermentative oral Lactobacilli isolated from rats, 3 groups were distinguishable, as shown by the characteristic shapes of their growth inhibition curves. The homofermentative Lactobacilli were divided into 2 groups: in one, typified by strain 84 AT, a monomolecular reaction was observed and in a second, illustrated by strain pp3, a resistance to sodium fluoride through 50 p.p.m. was noted. The third group, represented by strain 20, was composed of heterofermentative Lactobacilli. The growth rate of strain 84 AT was reduced by 50, 100, 200, and 300 p.p.m. NaF. At concentrations of 400 or 500 p.p.m., there was an apparent lag after which the rate of growth was approximately the same as that at 300 p.p.m. With strains pp3 and 20, rates of growth were decreased only slightly through 500 p.p.m. NaF. The effect of pH on the inhibition of growth by sodium fluoride indicated that as the pH of the growth medium was lowered from pH 8 through 4, the inhibitory action of sodium fluoride against strains 84 AT, pp3, and 20 was increased. In bactericidal experiments 99.9 per cent of cells of strain 84 AT were killed when they were exposed to 600 p.p.m. NaF for 2 hours at pH 6.9. The bactericidal effect of sodium fluoride was increased as the pH of the testing mixture was decreased from pH 8 to 4.