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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 …