Antimicrobial photodynamic therapy for burn wound infection with P. aeruginosa in rats

Pseudomonas (P.) aeruginosa is a typical Gram-negative bacterium to cause burn wound infection, which is hard to control since P. aeruginosa efficiently forms biofilms and easily acquires drug resistance, resulting in septicemia. Antimicrobial photodynamic therapy (aPDT) is a new approach for controlling such burn wound infection, and we showed a certain effectiveness of methylene blue (MB)-mediated aPDT with LED array illumination for a rat model with an extended, full-thickness burn infected with P. aeruginosa. However, rapid bacterial regrowth was observed even after consecutive daily aPDT, indicating efficient bacterial invasion into the tissue. Thus, it is crucial to evaluate the aPDT efficacy in a depthresolved manner. In this study, we analyzed depth distributions of bacteria for the rat treated with saline (control), rat treated with a photosensitizer mixture alone (PS alone) and rat treated with a PS mixture plus light (aPDT) based on Gramstained tissue sections for the same rat model as that described above. For the control rat, large number of bacteria were observed on the tissue surface and in the wide depth range of the skin. For the PS alone rat, many bacteria were observed on the tissue surface and in the epidermal region, but the number of bacteria was limited in the deeper region of the skin, indicating a PS mixture dark toxicity. The aPDT showed drastic decreases in the number of bacteria both on the tissue surface and in the subsurface region; however, there were still certain numbers of bacteria in the deeper region of the skin.