Dental caries is a dieto-bacterial disease that continues to affect the vast majority of people. At least three factors are essential for the establishment of the disease: a susceptible host, cariogenic micro-organisms and carbohydrate, in particular sugars.

Saliva has important properties, such as cleansing action, buffering capacity and antibacterial effects, that may modulate the occurrence of dental caries. Thus, impaired salivary-gland function, which is a common occurrence and is frequently associated with the use of several drugs, radiation treatment, and autoimmune disease, contributes to the development of severe oral disease, including caries. Furthermore, desalivated animals are highly susceptible to infection by Streptococcus sobrinus and rapidly develop extensive carious lesions, probably representing one of the most severe cariogenic challenges available in an experimental model.

Micro-organisms, colonizing tooth surfaces, form dental plaque through the synthesis of extracellular polysaccharides, ferment carbohydrates, mainly sugars, to organic acids that induce demineralization of teeth, resulting in caries. Investigations in animals and man show a highly positive correlation between frequency of ingestion of sugars and the incidence of caries. Besides the frequency of ingestion, the length of the intervals between exposures to carbohydrates influences the development of caries. Thus, it is clear that the intensity of cariogenic challenge can be influenced through several distinct mechanisms.

All the available evidence shows clearly that fluoride is an effective cariostatic agent. It exerts its protective effect by preventing demineralization and promoting remineralization of early carious lesions in teeth; it also inhibits some bacterial glycolytic enzymes.

The preventive effect of fluoride is critically dependent on its ambient concentrations in the mouth, even though the amount of fluoride required for optimal effect under various levels of cariogenic challenge has not been determined. The percentage of caries reduction by fluoride present at optimal concentration in drinking water in man varies from as low as a 10% to as high as 60%. Differences in the intensity of cariogenic challenge, for whatever reasons, may play a part in determining these fluctuations in the effectiveness of fluoride. Fluoride, like other therapeutic agents, is probably more effective on teeth subjected to a low rather than high level of cariogenic challenge. Nevertheless, usually just one level of exposure to fluoride is provided regardless of potential levels of disease.

Our purpose now was 2-fold. First, to determine the effect of different concentrations of fluoride in drinking water on caries development in desalivated rats fed ad libitum and also to examine whether there is a maximum protective effect that may be achieved with use of fluoride. Second, to explore the cariostatic effect of fluoride in animals exposed to different levels of cariogenic challenge, which were controlled by a programmed feeding machine.

Here we explored the effect of different concentrations of fluoride on the development of dental caries. Sixty female Sprague–Dawley sialodacryoadenitis virus-free rats, aged 12 days, were purchased with their dams from the Charles River (Kingston, NY, U.S.A.) facility. The dams, screened as described by Rosalen et al. (1997), were infected orally with an actively growing culture of Strep. sobrinus 6715, that had been re-isolated from a desalivated rat, fed laboratory chow, and given 5% sucrose water to drink ad libitum. The pups were reinfected when 20 and 21 days old, fed diet 2000 (Ziegler Brothers, Gardners, PA, U.S.A.) and given 5% sucrose water to drink ad libitum. The pups were weaned when aged 23 days and oral swabs from them were checked for infection by Strep. sobrinus on mitis salivarius agar (Difco, Detroit, MI, U.S.A.) plus streptomycin (Sigma). When aged 24 days, the pups were desalivated using our previously described technique. The pups were divided into five groups of 12.

In this part of the investigation, we explored the cariostatic effect of fluoride under controlled levels of cariogenic challenge. Eight litters of 10 female Sprague–Dawley pups, aged 13 days, were purchased together with their dams. The dams were screened and infected, and the initial procedures with the pups were the same as described for experiment I. When aged 25 days, the pups were desalivated, distributed into eight groups of nine and placed in a König–Höfer programmed feeder. Four groups received sterile distilled water and four groups received sterile distilled water containing 10 parts/10⁶ F ad libitum.

Essential nutrition (diet NCP No. 2) was provided by gavage twice daily. Animals were weighed weekly. At the end of the 21-day experimental period, the animals were killed by CO₂ asphyxiation and decapitated. In both experiments, the lower left jaw was aseptically removed and prepared for bacteriological examination as previously described. Smooth-surface and sulcal lesions were scored according to Larson’s modification of Keyes’s technique. Analysis of variance and the Tukey–Kramer HSD test were done with JMP version 2 software. Certain outcome measures were analysed with transformed values in order to stabilize variances; smooth-surface and sulcal caries scores were expressed as proportions of their maximum possible values (124 and 56, respectively), and the arcsine transformation was applied. The microbial counts were expressed as logarithms before analysis.

All animals remained healthy throughout experiment I. There was no difference in weight gain among the groups; the mean weight gain (SD) of all the groups was 54 (3.8) g.

Animals receiving fluoridated water developed fewer carious lesions than did the control group. Animals receiving 30 parts/10⁶ fluoride in drinking water showed the lowest smooth-surface caries scores, which did not significantly differ from those in the 40 parts/10⁶ fluoride group. All concentrations of fluoride were effective in decreasing the severity of smooth-surface lesions, and the 30 and 40 parts/10⁶ fluoride groups had the lowest severity scores. Caries does not affect all smooth surfaces equally, and therefore we examined the effect of fluoride on separate smooth surfaces. Buccal caries scores were lower in the animals that received fluoride, but no statistical difference was observed between the control and the 10 parts/10⁶ fluoride group. Fluoride was effective in decreasing lingual smooth-surface caries; all fluoride groups showed a lower score than the control group. The 30 and 40 parts/10⁶ fluoride groups showed statistically significant lower scores on approximal surfaces than the control.

There was essentially no difference in the sulcal caries scores of the control animals and those receiving drinking water containing 10 parts/10⁶ F. The 30 parts/10⁶ F group had the lowest sulcal caries score, but there was no statistical difference between the control and that group. The severity scores of the fluoride groups did not differ from those of the control group.

The control group harboured the highest number of total cultivable flora and was not statistically different from the 40 parts/10⁶ F group. Strep. sobrinus populations were significantly lower in the fluoride groups than in the control group, but the its populations in the various fluoride groups did not differ from each other statistically. The lowest percentage of Strep. sobrinus was observed in the 40 parts/10⁶ F group, but it was not statistically different from that in the 20 and 30 parts/10⁶ F groups.