Hydrogen Peroxide Enhances the Antibacterial Effect of Methylene Blue-based Photodynamic Therapy on Biofilm-forming Bacteria

Recently, increased attention has been focused on endoscopic disinfection after outbreaks of drug-resistant infections associated with gastrointestinal endoscopy. The aims of this study were to investigate the bactericidal efficacy of methylene blue (MB)-based photodynamic therapy (PDT) on Pseudomonas aeruginosa (P. aeruginosa), which is the major cause of drug-resistant postendoscopy outbreak, and to assess the synergistic effects of hydrogen peroxide addition to MB-based PDT on biofilms. In planktonic state of P. aeruginosa, the maximum decrease was 3 log ₁₀ and 5.5 log ₁₀ at 20 and 30 J cm ⁻² , respectively, following MB-based PDT. However, the maximum reduction of colony forming unit (CFU) was decreased by 2.5 log ₁₀ and 3 log ₁₀ irradiation on biofilms. The biofilm formation was significantly inhibited upon irradiation with MB-based PDT. When the biofilm state of P. aeruginosa was treated with MB-based PDT with hydrogen peroxide, the CFU was significantly decreased by 6 log ₁₀ after 20 J cm ⁻² , by 7 log ₁₀ after 30 J cm ⁻² irradiation, suggesting significantly higher efficacy than MB-based PDT alone. The implementation of the combination of hydrogen peroxide with MB-based PDT through working channels might be appropriate for preventing early colonization and biofilm formation in the endoscope and postendoscopy outbreak.