This research studied the effectiveness of the photoactive compound methylene blue (MB) activated with red LED light (576-672 nm) compared to that of caspofungin (CAS) on 1 Candida albicans and 3 Candida parapsilosis strains. Results were evaluated in terms of SMIC50 for CAS or in PDI (photodynamic inactivation)-SMIC50 for MB (minimal inhibitory concentration inhibiting sessile biofilm to 50% in comparison to the control without CAS or after irradiation in comparison to the control without MB). While all strains were susceptible to CAS in planktonic form, the SMIC50 was determined to be >16 μg mL(-1) when CAS was added to a 24 h biofilm. However, PDI-MIC50s (1.67 mW cm(-2) , fluence 15 J cm(-2) ) were 0.0075-0.03 mmol L(-1) . For biofilm, PDI-SMIC50s were in the range from 0.7 to 1.35 mmol L(-1) . MB concentration of 1 mmol L(-1) prevented a biofilm being formed ex vivo on mouse tongues after irradiation regardless of the application time, in contrast to CAS, which was only effective at a concentration of 16 μg mL(-1) when it was added at the beginning of biofilm formation. PDI seems to be a promising method for the prevention of microbial biofilms that do not respond significantly to conventional drugs.
© 2015 The American Society of Photobiology.