To evaluate the in vitro efficacy of rose bengal– and riboflavin-mediated photodynamic therapy for inhibition of methicillin-resistant Staphylococcus aureus (MRSA) isolates.

Experimental study.

Two different multidrug-resistant, clinical MRSA isolates were grown on nutrient agar, prepared in suspension, and adjusted to concentrations of 1.5 × 10⁴ colony-forming units per milliliter. Bacterial suspensions were mixed with rose bengal, riboflavin, or water according to experimental group. Tested in triplicate, groups included: Group I, MRSA control; Group II, MRSA with 0.1% rose bengal; Group III, MRSA with 0.03% rose bengal; and Group IV, MRSA with 0.1% riboflavin. All experimental groups were exposed to 3 lighting conditions: dark, ambient room light for 30 minutes, and 5.4 J/cm² with either green light-emitting diode (LED) or ultraviolet-A (UV-A) irradiation. Plates were photographed at 72 hours and custom software measured bacterial growth inhibition.

Complete growth inhibition of both MRSA strains was demonstrated (1) for both rose bengal concentrations under ambient and green LED irradiation, and (2) for the 0.1% rose bengal in the dark. The 0.03% rose bengal in dark conditions showed complete inhibition of strain 2 but incomplete inhibition of strain 1. Riboflavin showed almost complete inhibition with UV-A irradiation but demonstrated minimal inhibition for both strains in dark and ambient light conditions.

Rose bengal– and riboflavin-mediated photodynamic therapy demonstrated complete growth inhibition in vitro of 2 multidrug-resistant MRSA strains. Rose bengal was also effective in dark and ambient conditions. These results may have implications for in vivo therapy.