H₂O₂-producing commensal streptococci disrupt Streptococcus mutans–Candida albicans synergism

Microbial homeostasis is maintained by the antagonistic capacity of commensal bacteria against cariogenic pathogens. In the oral cavity, commensal Streptococcus, dominant colonizers of the tooth surface, can produce hydrogen peroxide (H₂O₂), modulating virulent cross-kingdom biofilm formation. To investigate their ecological role, clinical isolates from dental plaque were compared with reference strains, including Streptococcus oralis ATCC 35037 and S. oralis subsp. tigurinus J22 to determine their H₂O₂-producing capabilities. The antagonistic potential of S. oralis against Streptococcus mutans and Candida albicans was evaluated using microbial and biochemical assessments. In a saliva-coated hydroxyapatite disc model, S. oralis strains were co-cultured with S. mutans and C. albicans. A high H₂O₂-producing S. oralis J22 inhibited EPS formation in S. mutans and yeast-to-hypha transition in C. albicans, thereby reducing EPS-mediated bacterial–fungal cell colocalization. Time-lapse confocal imaging revealed that S. oralis J22 dominated the biofilm through H₂O₂-mediated antagonistic interactions. In contrast, the inhibitory effect of S. oralis strains lacking the spxB gene on cross-kingdom biofilms was significantly reduced. These data provide ecological insights into how physicochemical properties of early colonizing commensals shape the structure and virulence of cross-kingdom oral biofilm through antimicrobial-mediated antagonistic activity.