Effects of long-term fluoride mouthrinse use from early childhood on dental caries experience and salivary Streptococcus mutans in adolescents

Background: Fluoride mouthrinse (FMR) is an established and effective measure for preventing dental caries. However, its long-term effects on the oral microbial ecology remain insufficiently understood. This study aimed to investigate the association between long-term school-based FMR initiated in early childhood, dental caries experience, and the quantitative characteristics of salivary cariogenic bacteria in adolescents. Methods: A cross-sectional study was conducted among 251 adolescents aged 12–15 years attending two junior high schools in Japan. The intervention group (n = 72) had participated in a school-based FMR programme from preschool through primary school, whereas the control group (n = 179) had not. Dental caries experience in permanent teeth (decayed and filled teeth; DFT) was assessed. Salivary Streptococcus mutans and Streptococcus sobrinus were quantified by real-time polymerase chain reaction, and their relative abundance was calculated using the total bacterial load as the reference. Group comparisons and age-adjusted multivariable logistic regression analyses were performed. Results: The intervention group showed significantly lower mean DFT than the control group (0.43 ± 0.93 vs. 1.32 ± 2.22, p < 0.05), and a lower proportion of participants with dental caries experience (22.2% vs. 44.7%, p < 0.05). The relative abundance of salivary S. mutans was significantly lower in the intervention group (0.007 ± 0.015 vs. 0.015 ± 0.076, p < 0.05), although its detection rate was significantly higher (58.3% vs. 33.0%, p < 0.05). No significant differences were observed in the relative abundance or detection rate of S. sobrinus . After adjustment for age, the control group remained significantly more likely to have dental caries experience (adjusted odds ratio [aOR], 2.30; 95% confidence interval [CI], 1.19–4.44), whereas S. mutans detection was significantly less frequent (aOR, 0.43; 95% CI, 0.24–0.78). Conclusion: Long-term fluoride mouthrinse initiated in early childhood was associated with reduced dental caries experience and a lower relative abundance of salivary S. mutans in adolescence. These findings suggest that fluoride mouthrinse may contribute to caries prevention not by eliminating S. mutans , but by limiting its ecological predominance within the oral microbiota, supporting the contemporary ecological concept of dental caries.