Antimicrobial photodynamic activity of Rose Bengal, alone or in combination with Gentamicin, against planktonic and biofilm Staphylococcus aureus

Abstract Antimicrobial photodynamic therapy (aPDT) could constitute an alternative therapy to antibiotics especially against superficial infections caused by bacteria involved in multidrug resistance processes. The aim of this study is to compare the efficacy of aPDT using the photosensitizer rose bengal (RB), combined or uncombined with gentamicin (GN), against Staphylococcus aureus. Different concentrations of RB (ranging from 0.03 to 64 μg/ml) were added to S. aureus in water suspensions or forming biofilms in the absence or presence of GN (1–40 μg/ml) and the samples were irradiated (18 or 37 J/cm 2 ). The number of viable bacteria was quantified by counting colony-forming units. RB-aPDT shows significant photoactivity. The combination of GN and RB-aPDT exerts a synergistic bactericidal effect against planktonic S. aureus . On the other hand, a synergistic effect is observed only when the maximum concentration tested of RB and GN was used in biofilm. According to these result the use of RB-aPDT alone or in combination with GN could be implemented against S. aureus.