Photodynamic therapy using methylene blue, combined or not with gentamicin, against Staphylococcus aureus and Pseudomonas aeruginosa

Antimicrobial photodynamic therapy (a-PDT), combined or not with antibiotics, constitutes a promising therapy for superficial infections caused by bacteria implicated in multidrug resistance processes.We compared the efficacy of aPDT using the photosensitizer methylene blue (MB), combined or not with the antibiotic gentamicin (GN), against Staphylococcus aureus and Pseudomonas aeruginosa.Different concentrations of MB (0.03–7000 μg/mL), with or without GN (1–20 μg/mL), were added to planktonic cultures or biofilms and the samples irradiated with a LED lamp (λ 625 nm, 7 mW/cm2, 18 J/cm2). The number of viable bacteria in the samples and in corresponding nonirradiated controls was quantified by counting colony-forming units to evaluate the individual effects of MB, GN, and irradiation.MB-aPDT resulted in significant bacterial photoinactivation. The combination of GN and MB-aPDT exerted a synergistic bactericidal effect against planktonic cultures of S. aureus and P. aeruginosa. This combination did not significantly alter the photoinactivating effect of MB against S. aureus biofilms, but exerted a positive bactericidal effect against P. aeruginosa biofilms.These results underscore the need for further clinical studies of this therapeutic combination for the management of difficult-to-treat skin and mucous infections, especially those caused by P. aeruginosa.