Effect of a sodium fluoride rinse on the composition of early oral biofilms formed in situ on enamel and dentine

OBJECTIVES: Dental caries results from dysbiotic shifts in the microbial community, so modulating biofilm composition represents a valid preventive strategy. The aim of this double-blind randomized crossover in situ study was to investigate how a fluoride rinse can modify the composition of the early oral biofilm formed on enamel and dentine. MATERIALS AND METHODS: Twelve volunteers wore mandibular appliances containing 3 enamel and 3 dentine specimens for 1 min, to allow basal pellicle formation, then rinsed (1 min) with a NaF solution (500 ppm F⁻) or deionized water (DW) and kept the appliance overnight (8 h). The composition of the biofilms was analysed using full-length 16 S rRNA gene sequencing. RESULTS: Alpha and beta diversity did not differ between groups. Dominant genera were Streptococcus (61-71%), Haemophilus (15-24%), Gemella (4-6%), Veillonella (1-3%), and Rothia (1-2%), with no significant group differences. On both enamel and dentine, NaF rinse significantly increased the relative abundance of several Streptococcus spp., including S. salivarius and S. toyakuensis, while decreasing species of Streptococcus, Neisseria, and Prevotella. On enamel, NaF reduced S. gordonii and increased S. oralis and S. parasanguinis; on dentine, it increased species of Rothia and Haemophilus. CONCLUSIONS: NaF rinse induces subtle species-level changes in early biofilms formed on enamel and dentine, while overall community diversity and dominant genera remain stable. CLINICAL RELEVANCE: This exploratory study suggests that a 500 ppm NaF rinse is associated with subtle species-level shifts in early oral biofilms. Further studies are needed to determine whether these changes translate into functional benefits or improved oral health outcomes. CLINICAL TRIAL REGISTRATION: The study protocol was registered at ClinicalTrials.gov (NCT04033263).