The influence of diet, saliva, and dental history on the oral microbiome in healthy, caries-free Australian adults

Oral microbiome transplantation (OMT) has the potential to serve as a therapeutic approach for managing dental caries; however, it is essential to identify suitable donors. The aim of this study was to analyse the relationship between dietary (i.e., energy, water, carbohydrate and sugar intake), saliva quality (i.e., saliva flow rate and salivary pH), and clinical factors (i.e., past caries experience and fluoride exposure) on the oral microbiome composition of potential OMT donors. For this cross-sectional study, a comprehensive dental examination was conducted for 93 healthy, caries-free adults (aged 18–85 years) without systemic or oral disease. All participants completed questionnaires on food frequency and socio-economic, lifestyle, and behavioural factors. Supragingival plaque samples were collected, and bacterial 16S rRNA genes were amplified, sequenced, and assigned to bacterial taxa. Stimulated saliva samples were collected for salivary flow rate and pH measurements. Constrained partial ordination analysis revealed that dietary factors, such as carbohydrate and sugar intake, had strong directional influences on microbial composition, while salivary factors like flow rate and pH showed opposing effects. Correlation analysis linked high sugar intake and reduced salivary pH to increased Streptococcus abundance. Differential abundance analysis identified significantly higher abundance of Streptococcus species among low water and high carbohydrate and sugar consumers. In mediation analysis, sugar consumption was directly and indirectly linked to reduced salivary pH, with Streptococcus showing a significant negative mediation effect (mean: -0.198; 95% CI: -0.387 to -0.010). High carbohydrate and sugar intake significantly influenced alpha diversity metrics (p < 0.05). Beta diversity permutational multivariate ANOVA revealed that covariates explained 11.45–12.52% of microbial variation (p < 0.05). This study emphasises that OMT donors with diverse oral microbiomes, low sugar and carbohydrate intake, and reduced levels of acidogenic taxa, such as Streptococcus, which significantly mediate salivary pH reduction, may be preferred for caries prevention.