Inhibiting effects of <i>Streptococcus salivarius</i> on competence‐stimulating peptide‐dependent biofilm formation by <i>Streptococcus mutans</i>

<jats:p><jats:bold>Introduction: </jats:bold> The effects of <jats:italic>Streptococcus salivarius</jats:italic> on the competence‐stimulating peptide (CSP)‐dependent biofilm formation by <jats:italic>Streptococcus mutans</jats:italic> were investigated.</jats:p><jats:p><jats:bold>Methods: </jats:bold> Biofilms were grown on 96‐well microtiter plates coated with salivary components in tryptic soy broth without dextrose supplemented with 0.25% sucrose. Biofilm formations were stained using safranin and quantification of stained biofilms was performed by measuring absorbance at 492 nm.</jats:p><jats:p><jats:bold>Results: </jats:bold> <jats:italic>S. mutans</jats:italic> formed substantial biofilms, whereas biofilms of <jats:italic>S. salivarius</jats:italic> were formed poorly in the medium conditions used. Furthermore, in combination cultures, <jats:italic>S. salivarius</jats:italic> strongly inhibited biofilm formation when cultured with <jats:italic>S. mutans.</jats:italic> This inhibition occurred in the early phase of biofilm formation and was dependent on inactivation of the CSP of <jats:italic>S. mutans</jats:italic>, which is associated with competence, biofilm formation, and antimicrobial activity of the bacterium, and is induced by expression of the <jats:italic>comC</jats:italic> gene. Comparisons between the <jats:italic>S. mutans</jats:italic> clinical strains FSC‐3 and FSC‐3Δ<jats:italic>glrA</jats:italic> in separate dual‐species cultures with <jats:italic>S. salivarius</jats:italic> indicated that the presence of the bacitracin transport ATP‐binding protein gene <jats:italic>glrA</jats:italic> caused susceptibility to inhibition of <jats:italic>S. mutans</jats:italic> biofilm formation by <jats:italic>S. salivarius</jats:italic>, and was also associated with the regulation of CSP production by <jats:italic>com</jats:italic> gene‐dependent quorum sensing systems.</jats:p><jats:p><jats:bold>Conclusion: </jats:bold> It is considered that regulation of CSP by <jats:italic>glrA</jats:italic> in <jats:italic>S. mutans</jats:italic> and CSP inactivation by <jats:italic>S. salivarius</jats:italic> are important functions for cell‐to‐cell communication between biofilm bacteria and oral streptococci such as <jats:italic>S. salivarius</jats:italic>. Our results provide useful information for understanding the ecosystem of oral streptococcal biofilms, as well as the competition between and coexistence of multiple species in the oral cavity.</jats:p>