Surfactin-methylene blue complex under LED illumination for antibacterial photodynamic therapy: Enhanced methylene blue transcellular accumulation assisted by surfactin

Recently, increased attention has been focused on antibacterial photodynamic therapy (APDT) to treat multidrug-resistant bacterial infection due to the antibiotic abuse. Methylene blue has been used as a kind of efficient and cheap commercial photosensitizer in APDT. However, due to high hydrophilicity, methylene blue is not able to be transcellular intaken and accumulated efficiently. To promote accumulation and APDT efficiency of methylene blue, lipopeptide surfactin-methylene blue complex has been prepared through electrostatic interaction. The complex under LED irradiation was found to effectively reduce 5.0 Log₁₀ CFU and 7.6 Log₁₀ CFU for P. aeruginosa and S. aureus, respectively. The bacterial reduction efficiency is slightly higher than free methylene blue. The photosensitizers accumulation and APDT targeting protein have been characterized by fluorescence spectroscopy, fluorescence microscopy and protein electrophoresis techniques. These results demonstrated that more surfactin-methylene blue complex could be accumulated more into the cell, and inactivate bacteria through destroying intracellular protein under LED illumination. In comparison, free methylene blue under light could inactivate bacteria through destroying membrane protein and lipid structures. These results would provide valuable insight for developing advanced clinical medicine and designing photo-drug for photodynamic therapy.