Photodynamic inactivation mediated by methylene blue or chlorin e6 against Streptococcus mutans biofilm

•The antimicrobial efficacy of MB did not increase with increasing concentration.•The antimicrobial efficacy of Ce6 increased with increasing concentration.•Low dark toxicity on S. mutans biofilms can be observed for Ce6.•Ce6 showed stronger antimicrobial efficacy than MB at higher concentrations. An appropriate photosensitizer (PS) for photodynamic inactivation should have a pronounced antimicrobial efficacy but low dark toxicity. The aim of this study is to investigate the concentration-dependent antimicrobial efficacies of methylene blue (MB) and chlorin e6 (Ce6), against Streptococcus mutans biofilms and to compare the efficacies of these two PSs. The 48-h S. mutans UA159 biofilms, grown on glass coverslips, were subjected to MB or Ce6 at 25, 50, 100 and 200 μM with or without irradiation by 660 nM LED light (L). Control groups (-PS-L and -PS + L) were also included. Viability of the biofilm was analyzed by CFU/biofilm and biofilm lactic acid production was quantified by an enzymatic assay. With irradiation, MB under 25 μM resulted in 2-log reduction in biofilm viability and 30-fold reduction in biofilm lactic acid production. However, this biofilm killing efficacy did not change with increasing MB concentration. The biofilm killing efficacy of Ce6 increased with increasing Ce6 concentrations and resulted in 5-log reduction in biofilm viability. The lactic acid inhibitory effect of Ce6 was significantly lower than MB at 25 μM (p