The effect of probiotic bacteria on community composition, microscale pH and matrix architecture in a saliva-derived model of oral biofilm

Background: Streptococcus salivarius K12 and M18 have recently gained interest as probiotic organisms for caries control. This study investigated the impact of S. salivarius K12 and M18 supplementation on biofilm virulence in a complex saliva-derived in vitro model compared to treatment with Limosilactobacillus reuteri DSM17938 and no treatment. Methods: Biofilms were grown under low (0.05%) and high (1%) sucrose conditions and supplemented with S. salivarius K12, S. salivarius M18 or L. reuteri. Microscale pH dynamics were assessed using confocal microscopy-based pH ratiometry. The biofilm matrix composition was analyzed by glucan and extracellular DNA mapping. Microbial community composition was determined by 16S rRNA gene amplicon sequencing. Results: At 0.05% sucrose, pH was significantly higher in S. salivarius M18 biofilms, but not in S. salivarius K12 biofilms, compared to control. pH in L. reuteri biofilms was significantly lower. At 1% sucrose, no significant effects of probiotic supplementation on biofilm pH were observed. Shifts in the bacterial composition were minor, most notably an increased abundance of lactobacilli in L. reuteri-treated biofilms at 1% sucrose. No significant effects on the biofilm matrix composition were observed. Conclusions: In conclusion, probiotic supplementation exerted only minor effects on biofilm composition and virulence.