pH-dependent bactericidal effects of acidulated fluoride gels on preformed plaque aggregates of Streptococcus mutans 6715

Abstract

The anticaries action of topically applied sodium fluoride (NaF) preparations has been attributed, at least in part, to the antibacterial effect of fluoride on cariogenic bacteria. In this report we examined the bactericidal activity of acidulated NaF gels against preformed plaque aggregates of Streptococcus mutans 6715. Both commercially available and laboratory-prepared gels were tested. Prepared NaF gels were acidified to various pH levels with HCI, H3PO4, or HF. The aggregates of S. mutans were immersed in the various fluoride gels for 5 min and then monitored for viability. None of the commercially available NaF gels with pHs of 3.5 to 4.5 and fluoride concentrations of 5,000 to 12,300 ,ug/ml were capable of exerting complete bactericidal activity against aggregates of S. mutans. For the laboratory-prepared NaF gels, neither the source of hydrogen ion nor the source of fluoride influenced the outcome when fluoride was present at the appropriate MBC and pH. NaF gels with a fluoride concentration of 37,000 ,ug/ml were bactericidal to S. mutans at pH 3.0. At pH 2.5 and 2.0, approximately 12,000 and 1,100 ,ug of fluoride per ml, respectively, was bactericidal. Results from this study suggest that the bactericidal properties of acidulated NaF gels can be enhanced by lowering their pHs to below 3.0.Frequent and infrequent topical applications of high concentrations of fluoride (>1,000 ,ug/ml) inhibit dental caries (for review, see reference 6). The exact mechanism(s) responsible for this inhibition, however, remains unclear. Several studies have reported reductions in proportions of cariogenic bacteria such as Streptococcus mutans for various intervals after topical fluoride applications, suggesting that fluoride, when present in these high concentrations, might exert marked inhibitory and even bactericidal effects on these bacteria (2,16,17, 26).Attempts have been made to examine the antibacterial properties of fluoride on cariogenic bacteria with various in vitro assays. For example, broth dilution (2,4,8,18) and pellet culture (19) assays clearly indicate that fluoride is capable of exerting both inhibitory and bactericidal effects on strains of S. mutans, depending upon the concentrations of the organisms and fluoride, duration of exposure, and pH of the medium, among other factors. Unfortunately, these conventional in vitro methods tend to greatly overestimate the antimicrobial potency of fluoride when compared with what can be expected in vivo when factors such as density of the microbial mass colonizing the teeth, accessibility of the agent to various tooth surfaces, and duration and frequency of applications are considered (24). More specifically, conventional assays involve exposing dispersed bacterial suspensions to antimicrobial agents for prolonged periods (i.e., 18 to 48 h) and usually in the presence of nutrient medium, which can inactivate or adversely affect the properties of the antimicrobial agent. Consequently, extrapolating from these observations to predict what will occur in vivo can be misl...

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