The Effects of Potassium Fluoride and Potassium Laurate on pH Gradients inStreptococcus downeii

Inhibition of acid production in Streptococcus mutans R9: inhibition constants and reversibility

End-product inhibition of acid production in Streptococcus mutans R9 was investigated by measuring effects of varying concentrations of H+ and of undissociated lactic or acetic acids on acid production rates in the pH stat. H+ caused purely uncompetitive inhibition (inhibition constant Kiu 0.018 mmol 1(-1). Lactic acid caused mixed inhibition with inhibition constants of Kiu 4.24 mmol 1(-1) and Kic 4.55 mmol 1(-1). Reversibility of inhibition by H+ showed only a statistically significant reduction only at pH < 4.5. Reversibility of inhibition by lactic and acetic acids decreased with increasing inhibitor concentration. In all cases, reversibility correlated with the extent to which viability was retained, suggesting that loss of reversibility was due to cell death. These results suggest that, after a low-pH episode in dental plaque, caused by fermentation of dietary carbohydrate, the ability of plaque organisms to produce acid in subsequent exposures to carbohydrate may be reduced.

Effect of sucrose monolaurate on acid production, levels of glycolytic intermediates, and enzyme activities of Streptococcus mutans NCTC 10449

We studied the mechanism by which the antimicrobial compound sucrose monolaurate inhibits Streptococcus mutans NCTC 10449 by determining its effect on the rate of acid production from glucose and sucrose and the intracellular and extracellular levels of glycolytic intermediates. Sucrose monolaurate was more effective than either sodium laurate or sodium fluoride in inhibiting acid production at pH 7.0 from glucose. Inhibition of acid production was the same when either glucose or sucrose was the carbon source and in the presence or absence of oxygen. Quantitative analysis of various glycolytic intermediates revealed that the steps inhibited by sucrose monolaurate were the reactions catalyzed by phosphofructokinase and glyceraldehyde 3-phosphate dehydrogenase and/or phosphoglycerate kinase. Since the activities of these enzymes in cell-free extracts were not decreased by the addition of sucrose monolaurate, the inhibition of acid production could not be ascribed to direct effects on the enzymes. A decrease in the rate of acid production with corresponding elevations in the extracellular levels of glycolytic intermediates indicates that sucrose monolaurate inhibits S. mutans by altering the permeability of the cell membrane, which causes a loss of important metabolites.

Effects of low fluoride concentrations on formation of caries-like lesions in human enamel in a sequential-transfer bacterial system

Human enamel surfaces were exposed to sequential batch cultures of Streptococcus mutans NCTC 10832 in a sucrose-rich medium containing 0-5 mg/l added fluoride (F). In 10-day experiments, subsurface lesion formation was partly inhibited by 1 mg/l F and completely by 2 and 5 mg/l F, but small lesions formed in 2 mg/l F in 21-day experiments. Analysis of the spent media, together with analogous, bacteria-free experiments, suggested that lesion inhibition involved two main effects. First, inhibition of bacterial acid production reduced the pH fall, resulting in reduced undersaturation with respect to hydroxyapatite and consequently reduced rate of demineralization. Secondly, interaction of F with enamel mineral resulted in a small increase in reprecipitation during periods of supersaturation and a much larger reduction in demineralization during periods of undersaturation. It is concluded that, at low F concentrations, inhibition of bacterial acid production is a major factor in lesion inhibition, which may contribute significantly to caries prevention in vivo where plaque fluid F levels are raised by frequent topical applications.

pH regulation by Streptococcus mutans

The intracellular pH (pHi) optimum for glycolysis in Streptococcus mutans Ingbritt was determined to be 7.0 by use of the ionophore gramicidin for manipulation of pHi. Glycolytic activity decreased to zero as the pHi was lowered from 7.0 to 5.0. In contrast, glycolysis had an extracellular pH (pHo) optimum of 6.0 with a much broader profile. The relative insensitivity of glycolysis to the lowering of pHo was attributed to the ability of S. mutans to maintain a transmembrane pH gradient (delta pH, inside more alkaline) at low pHo. At a pHo of 5.0, glycolyzing cells of S. mutans maintained a delta pH of 1.37 +/- 0.09 units. The maintenance of this delta pH was dependent on the concentration of potassium ions in the extracellular medium. Potassium was rapidly taken up by glycolyzing cells of S. mutans at a rate of 70 nmol/mg dry weight/min. This uptake was dependent on the presence of both ATP and a proton motive-force (delta p). The addition of N-N'-dicyclohexylcarbodiimide (DCCD) to glycolyzing cells of S. mutans caused a partial collapse of the delta pH. Growth of S. mutants at pHo 5.5 in continuous culture resulted in the maintenance of a delta pH larger than that produced by cells grown at pH 7.0. These results suggest the presence of a proton-translocating F1Fo-ATPase in S. mutans whose activity is regulated by the intracellular pH and transmembrane electrical potential (delta psi). The production of an artificial delta p of 124 mV across the cell membrane of S. mutans did not result in proton movement through the F1Fo-ATPase coupled to ATP synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of medium chain fatty acids and fluoride on fissure caries in Wistar rats

This work was to compare the degree of caries protection provided by fatty acids with chain lengths of 9-12 carbons and to assess the anti-caries potential of a fluoride-fatty acid mixture. Male Wistar rats were weaned at 21 days, maintained on a standard laboratory diet for 7 days and then provided with a cariogenic, 45% sucrose diet for 34 days. This regimen produced a very reproducible degree of fissure caries and the inclusion of 1% powdered potassium nonanoate or decanoate in the diet gave large and significant reductions in total and advanced lesions. Decanoate was significantly more inhibitory towards advanced lesions than was nonanoate. In contrast, 1% potassium laurate was considerably less effective under these conditions. Caries protection with a mixture of 0.25% decanoate and 76 parts/10(6) fluoride was found to be cumulative rather than synergistic. It is suggested that the optimal effect obtained with decanoate was due to a favourable combination of the surfactant and wetting properties of this molecule at the low pH values found in fissure dental plaque. It is also suggested that fluoride and decanoate acted in an independent manner to enhance remineralization of early lesions and to inhibit the acidogenic flora, respectively.