Streptococcus mutans biofilm formation is dependent on extracellular DNA in primary low pH conditions

Objective Streptococcus mutans is an etiological agent of human dental caries and produces glucosyltransferase (GTF), encoded by genes gtfB and gtfC, an enzyme that synthesizes insoluble glucan from sucrose to form sticky biofilms. However, GTF functions are limited by pH. To observe the effects of low pH in primary culture conditions, we performed biofilm formation assays with S. mutans and compared the results obtained at pH 6.0 and pH 7.0. Methods Biofilm formation of S. mutans MT8148 was observed after 16 h in 96-well microtiter plates containing Tryptic soy broth (TSB) without dextrose and with 0.25% sucrose. Roles of extracellular DNA (eDNA) in biofilm formation, with and without insoluble glucans, were also examined by addition of DNase and colony stimulating peptide (CSP) to the biofilm and gtfB mutant. Results S. mutans formed significant biofilms in the presence of eDNA after 16 h culture at pH 6.0, which showed lower production of insoluble glucans than culture at pH 7.0. Moreover, the S. mutans MT8148 gtfB mutant produced significant biofilms in pH 6.0 condition, and biofilm formation was inhibited by DNase and stimulated by addition of CSP. Insoluble glucan-independent and eDNA-dependent biofilm formation was observed in the stress condition of low pH. Conclusions Insoluble glucan-independent biofilm formation by S. mutans was significantly influenced by increasing eDNA in primary low pH culture conditions.