Dental caries arises from dysbiotic shifts in the oral microbial community, so interventions that modulate biofilm composition are of preventive interest. This double-blind, randomized crossover in situ study investigated whether a single rinse with sodium fluoride (NaF) at 500 ppm F– modifies the composition of very early oral biofilms formed on human enamel and dentine.
Twelve adult volunteers participated in the trial. The design was randomized, double-blind, and crossover, allowing each volunteer to serve as their own control. The study protocol was approved by the local ethics committee (Project ID 2022 − 01904), was registered at ClinicalTrials.gov (NCT04033263), and participants provided written informed consent.
Mandibular intraoral appliances carried three enamel and three dentine specimens per volunteer. After inserting the appliance, specimens were exposed for 1 minute to permit basal salivary pellicle formation. Participants then rinsed for 1 minute with either a NaF solution containing 500 ppm fluoride or with deionized water (control). Appliances were kept in place overnight (approximately 8 hours) to allow formation of early biofilm.
Biofilm composition was profiled using full-length 16S rRNA gene sequencing to resolve taxa to species level where possible. The analysis included measures of alpha diversity (within-sample diversity) and beta diversity (between-sample community differences), and differential abundance testing at genus and species resolution.
Alpha diversity and beta diversity metrics did not show significant differences between the NaF and control (deionized water) conditions. In other words, overall community richness and between-sample compositional differences remained stable after the single 500 ppm NaF rinse.
Across enamel and dentine specimens, the dominant genera detected in the early biofilms were Streptococcus (61–71% relative abundance), Haemophilus (15–24%), Gemella (4–6%), Veillonella (1–3%), and Rothia (1–2%). No significant differences in the relative abundance of these dominant genera were reported between treatment groups.
Although overall diversity and dominant genera were stable, NaF was associated with subtle but significant species-level shifts. Several Streptococcus species increased in relative abundance after the NaF rinse, including S. salivarius and S. toyakuensis. Conversely, other Streptococcus species, as well as species of Neisseria and Prevotella, decreased in relative abundance following NaF exposure.
The species-level effects differed by substrate. On enamel specimens, NaF rinse reduced the relative abundance of S. gordonii while increasing S. oralis and S. parasanguinis. On dentine specimens, NaF was associated with increases in species of Rothia and Haemophilus. These findings indicate that the NaF-associated compositional shifts in very early biofilm differ between enamel and dentine surfaces.
A single 1-minute rinse with 500 ppm NaF before overnight biofilm formation produced modest, species-level alterations in early oral biofilms formed in situ on enamel and dentine, while leaving overall community diversity and dominant genera largely unchanged. The changes were subtle and surface-dependent, affecting specific Streptococcus species and select members of Neisseria, Prevotella, Rothia, and Haemophilus.
This exploratory study suggests that a routine-strength NaF rinse can modulate early biofilm composition at the species level, but it did not evaluate whether these compositional shifts lead to functional microbiome changes or measurable clinical benefits such as reduced cariogenicity. The authors state that further studies are required to determine whether the observed species-level changes translate to improved oral health outcomes.
The study followed the Declaration of Helsinki and was approved by a local ethics committee. It was registered on ClinicalTrials.gov (NCT04033263). The authors disclosed that one author (R.J.W.) recused from editorial handling due to an editorial role; the remaining authors declared no conflicts of interest.