Parameter-Dependent Effects of Rose Bengal Photodynamic Antimicrobial Therapy on MRSA Inhibition and Corneal Cytokine Expression

Purpose: To assess the antimicrobial effect of rose bengal and riboflavin photodynamic antimicrobial therapy (PDAT) on clinical methicillin-resistant Staphylococcus aureus (MRSA) isolates under varying photosensitizer concentrations, light conditions, and exposure times. Additionally, a pilot study was performed to evaluate cytokine responses in a human cornea epithelium model. Methods: Suspensions of six clinical MRSA isolates (four community and two health care associated) were prepared at 1.5 × 10⁶ colony-forming units/mL with different concentrations of rose bengal (0.2%/0.1%/0.05%/0.025%/0.0125%), riboflavin (0.2%/0.1%/0.05%), or water (control). Suspensions were plated on blood agar and exposed to four light conditions: dark, ambient light (15/30 minutes), or light-emitting diode irradiation (rose bengal = green, riboflavin = UV-A). Plates were incubated for 48 hours, and growth inhibition was assessed. IL-1β, IL-6, and IL-8 expression in corneal epithelial models was measured after exposure to MRSA and rose bengal PDAT. Results: All six MRSA strains were completely inhibited (100%) by PDAT with all rose bengal concentrations. Ambient light displayed variable inhibition, more pronounced at higher photosensitizer concentrations, while dark conditions produced no effect. Riboflavin did not inhibit MRSA. Cytokine responses were highest for IL-8, followed by IL-6. Rose bengal PDAT modulated the cytokine response. Conclusion: Rose bengal PDAT is an effective antimicrobial strategy against both community- and health care-associated MRSA. Rose bengal with ambient light is not recommended due to inconsistent efficacy in tested conditions. Riboflavin PDAT is ineffective under these conditions. MRSA increased cytokine production in corneal epithelial cells, and this response was modulated by PDAT. Translational Relevance: Rose bengal PDAT effectively suppresses MRSA and modulates inflammatory cytokines, supporting its use in microbial keratitis.