Surfactin selectively suppresses acidogenicity in Streptococcus sobrinus without inhibiting growth or biofilm formation

OBJECTIVES: Dental caries are caused by organic acids produced by cariogenic bacteria through carbohydrate metabolism. Suppression of acid production without disrupting the oral microbiome is a promising preventive strategy against dental caries. Surfactin, a naturally derived biosurfactant, has several biological activities. However, its effects on acid production by cariogenic bacteria remain unclear. In this study, the effects of surfactin on lactate production, growth, biofilm formation, and metabolic activity of Streptococcus sobrinus, were investigated. METHODS: In vitro assays were performed to distinguish surfactin-mediated suppression of acidogenic metabolism from its effects on bacterial growth or biofilm formation, combined with molecular and enzymatic analyses to explore the underlying regulatory mechanisms. RESULTS: Surfactin significantly reduced lactate production in planktonic and biofilm-associated S. sobrinus, and it delayed environmental pH reduction in the presence of sucrose. Notably, these effects were observed without inhibition of bacterial growth or biofilm formation. There were no significant changes in the expression of lactate production-related genes, and lactate dehydrogenase activity was not inhibited by surfactin. In contrast, in the MTT assay, there was a transient reduction in metabolic activity, accompanied by delayed initiation of growth. CONCLUSION: These findings indicate that surfactin selectively attenuates acidogenicity in S. sobrinus, without markedly affecting bacterial viability or biofilm architecture, which is consistent with an anti-virulence mode of action. Although further validation in more complex oral environments and comprehensive safety assessments are required, this study provides fundamental evidence supporting the potential of naturally derived biosurfactants as a basis for future preventive strategies for caries.