Streptococcus thermophilus DM287 and DM294 as candidate oral probiotic strains with anti-biofilm activity against cariogenic pathogens.

Streptococcus mutans and Streptococcus sobrinus are key contributors to dental caries within a broader polymicrobial biofilm community, driving pathogenicity through biofilm and dental plaque formation rather than direct tissue invasion. Probiotic-based strategies that target colonization rather than viability represent a promising non-antibiotic approach to caries prevention. Streptococcus thermophilus is a food-grade organism with established GRAS status and a long history of safe human consumption, yet it remains largely unexplored as an oral care ingredient. In this study, two S. thermophilus strains - DM287, isolated from a commercially available yogurt product, and DM294, isolated from the tongue coat of healthy adults - were characterized for anti-cariogenic activity through mechanisms independent of major pathogen abundance reduction. Both strains reduced plaque mass by approximately 75%-80% in a wire model assay and suppressed S. mutans and S. sobrinus biofilm biomass to near-baseline levels, while planktonic growth suggested that the observed effects were not primarily driven by bactericidal activity. Adhesion inhibition assays showed 32%-64% reduction in pathogen adhesion under protection conditions and 72%-76% under displacement conditions. Competitive colonization assays demonstrated that both strains increased surface attachment in pathogen-conditioned environments (fold change > 1.0), consistent with competitive displacement behavior. RT-qPCR analysis of S. mutans biofilms co-cultured with either strain revealed significant downregulation of gtfB and gtfC - key glucosyltransferase genes encoding the EPS synthetic machinery central to biofilm structure - with reductions of approximately 34%-63% depending on the strain and time point, suggesting transcriptional suppression of biofilm-associated virulence as a contributing mechanism. EPS quantification by phenol-sulfuric acid assay further demonstrated that both strains reduced EPS production by 80%-90%, a magnitude comparable to chlorhexidine and substantially exceeding that of Lacticaseibacillus rhamnosus GG. Taken together, these findings suggest that S. thermophilus DM287 and DM294 are biocompatible candidate strains with promising anti-biofilm potential for oral care applications, pending further validation.