Effects of fluorides on in vitro acid production by dental plaque

Effect of sodium fluoride on oral biofilm microbiota and enamel demineralization

OBJECTIVE

Fluoride is widely used as an anti-caries agent, e.g. in toothpastes and mouth rinses. However, the nature of the anti-caries action is not entirely clear. Mechanisms suspected to explain the cariostatic effect include inhibitory effects on acid formation by bacteria, inhibition of extracellular polysaccharide (EPS) production, inhibition of enamel demineralization and enhancement of remineralizaton or combination thereof. The aim of this study was to examine with the supragingival Zurich in vitro biofilm model the effect of fluoride in NaF formulation, on the microbiota and on demineralization.

METHODS

Biofilms consisting of Actinomyces oris, Candida albicans, Fusobacterium nucleatum, Streptococcus oralis, Veillonella dispar and Streptococcus sobrinus, were grown anaerobically on sintered hydroxyapatite or bovine enamel disks, exposed to 200, 400, and 1400 ppm of NaF, or 0.1% chlorhexidine (positive control). The biofilms were harvested after 64 h and CFUs were assessed for total bacteria. Demineralization of enamel disks was measured by quantitative light-induced fluorescence.

RESULTS

NaF did not affect the bacterial numbers. No enamel mineral loss was observed at 1400 and 400 ppm of fluoride, whereas the pH of the surrounding medium was increased to 5.5 and 5.0, respectively, compared to the untreated control (pH 4.5 and mineral loss ΔF of -32%). At 1400 ppm NaF the biofilm's EPS volume was also significantly reduced.

CONCLUSIONS

Administration of NaF completely prevented demineralization without affecting biofilm composition and growth. This protective effect may be attributed to the observed decrease in acid production or EPS volume, or to a shift in the de/remineralization balance.

Inhibition of multispecies biofilms by a fluoride-releasing dental prosthesis copolymer

OBJECTIVES

This study aimed to develop a new mixed-species acidogenic biofilm model and use it to assess the antimicrobial properties of a novel fluoride-releasing copolymer.

METHODS

Stubs composed of a copolymer of methyl methacrylate (MMA) and 2-hydroxyethyl methacrylate (HEMA) with polymethyl methacrylate (PMMA) were produced by chemically-activated free radical polymerization. A fluoride-releasing copolymer was developed by incorporating sodium fluoride in place of a portion of the PMMA. Samples were mounted in polysulfone Modified Robbins Devices (MRDs) and were optimized for single- and mixed-species biofilm formation by Candida albicans, Lactobacillus casei and Streptococcus mutans.

RESULTS

Fluoride release was sustained for at least 48h in flowing conditions. Fluoride did not affect the colonization and biofilm growth of any of the microorganisms in monocultures. However, in mixed-species biofilms, cell densities of all three species were reduced approximately ten-fold (p<0.05) on the fluoridated material compared with the non-fluoridated copolymer.

CONCLUSIONS

These data demonstrate that intermicrobial interactions in mixed-species acidogenic biofilms are sensitive to fluoride, and that the inclusion of fluoride in a denture lining copolymer reduces the formation of polymicrobial biofilms.

CLINICAL SIGNIFICANCE

The growth of acidogenic microorganisms on denture materials is associated with denture stomatitis and dental caries on surrounding teeth. A fluoride-releasing copolymer that inhibits acidogenic mixed-species biofilms, such as the material described in this study, has the potential to control these diseases by limiting biofilm growth.

Simultaneous Assessment of Acidogenesis-Mitigation and Specific Bacterial Growth-Inhibition by Dentifrices

Dentifrices can augment oral hygiene by inactivating bacteria and at sub-lethal concentrations may affect bacterial metabolism, potentially inhibiting acidogenesis, the main cause of caries. Reported herein is the development of a rapid method to simultaneously measure group-specific bactericidal and acidogenesis-mitigation effects of dentifrices on oral bacteria. Saliva was incubated aerobically and anaerobically in Tryptone Soya Broth, Wilkins-Chalgren Broth with mucin, or artificial saliva and was exposed to dentifrices containing triclosan/copolymer (TD); sodium fluoride (FD); stannous fluoride and zinc lactate (SFD1); or stannous fluoride, zinc lactate and stannous chloride (SFD2). Minimum inhibitory concentrations (MIC) were determined turbidometrically whilst group-specific minimum bactericidal concentrations (MBC) were assessed using growth media and conditions selective for total aerobes, total anaerobes, streptococci and Gram-negative anaerobes. Minimum acid neutralization concentration (MNC) was defined as the lowest concentration of dentifrice at which acidification was inhibited. Differences between MIC and MNC were calculated and normalized with respect to MIC to derive the combined inhibitory and neutralizing capacity (CINC), a cumulative measure of acidogenesis-mitigation and growth inhibition. The overall rank order for growth inhibition potency (MIC) under aerobic and anaerobic conditions was: TD> SFD2> SFD1> FD. Acidogenesis-mitigation (MNC) was ordered; TD> FD> SFD2> SFD1. CINC was ordered TD> FD> SFD2> SFD1 aerobically and TD> FD> SFD1> SFD2 anaerobically. With respect to group-specific bactericidal activity, TD generally exhibited the greatest potency, particularly against total aerobes, total anaerobes and streptococci. This approach enables the rapid simultaneous evaluation of acidity mitigation, growth inhibition and specific antimicrobial activity by dentifrices.

Effects on demineralization of enamel by fluoridated sucrose: a pilot study in an in situ caries model

Blocks of human enamel, placed in removable partial dentures, were allowed to acquire natural plaque for seven days and were exposed extra-orally to a cariogenic challenge by repeated periods in a fluoride-sucrose solution. As a control, enamel blocks were exposed extra-orally to a sucrose solution. After two weeks of cariogenic challenge, the blocks were examined for mineral loss by quantitative microradiography on thin sections of the enamel. The results from six subjects showed that no significant effect on demineralization could be detected by the addition of fluoride corresponding to fluoride/sugar content of 1 mg/kg, 5 mg/kg, or 10 mg/kg (dry weight). One subject did not develop lesions at all, either with fluoride-sucrose or with sucrose exposure alone.

The antimicrobial action of fluoride and its role in caries inhibition

Despite a considerable amount of literature on the effects of fluoride in dental plaque, several urgent questions remain unanswered, such as: Does the inhibiting effect of fluoride on dental plaque metabolism contribute to caries prevention? Does adaptation of plaque to fluoride affect its cariogenicity? Single applications of fluoride directly to dental plaque reduced acid production. Also, fluoride dissolving from topically treated enamel reduced the acid production in covering layers of oral bacteria in vitro. The effects of both treatments were only of short duration and may not be relevant to caries prevention in vivo. In contrast, daily applications of fluoride resulted in a reduction of the acidogenicity of dental plaque even 8-12 h after the treatment. Such a reduction is likely to contribute to caries prevention. But it has to be realized that when plaque reaches saturation with respect to fluoridated calcium (phosphate) precipitates, enamel becomes insoluble and any antimicrobial effect becomes irrelevant. Still lacking are data on the antimicrobial effects of fluoride regimens normally used in home care, in weekly rinsing programs in schools, or treatments applied professionally every six months. Adaptation of Streptococcus mutans to fluoride has been suggested to reduce the cariogenic potential of the cells. In vitro-induced fluoride-resistant strains were less cariogenic in rats, and the velocity of acid production in vitro was reduced at constant pH greater than 5.5. Despite the ability of oral bacteria to adapt to fluoride, evidence of adaptation in dental plaque of normal subjects resulting in a reduced cariogenic potential has not yet been demonstrated.