Photodynamic inactivation of Acinetobacter baumannii using phenothiazinium dyes: In vitro and in vivo studies

Background and Objective: Phenothiazinium dyes have been reported to be effective photosensitizers inactivating a wide range of microorganisms in vitro after illumination with red light. However, their application in vivo has not extensively been explored. This study evaluates the bactericidal activity of phenothiazinium dyes against multidrug-resistant Acinetobacter baumannii both in vitro and in vivo. Study Design/Materials and Methods: We report the investigation of toluidine blue O, methylene blue, 1,9-dimethylmethylene blue, and new methylene blue for photodynamic inactivation of multidrug-resistant A. baumannii in vitro. The most effective dye was selected to carry out in vivo studies using third-degree mouse burns infected with a bioluminescent A. baumannii strain, upon irradiation with a 652nm noncoherent light source. The mice were imaged daily for 2 weeks to observe differences in the bioluminescence-time curve between the photodynamic therapy (PDT)-treated mice in comparison with untreated burns. Results: All the dyes were effective in vitro against A. baumannii after 30 J/cm² irradiation of 635 or 652nm red light had been delivered, with more effective killing when the dye remained in solution. New methylene blue was the most effective of the four dyes, achieving a 3.2-log reduction of the bacterial luminescence during PDT in vivo after 360 J/cm² and an 800 μM dye dose. Moreover, a statistically significant reduction of the area under the bioluminescence-time curve of PDT-treated mice was observed showing that the infection did not recur after PDT. Conclusions: Phenothiazinium dyes, and especially new methylene blue, are potential photosensitizers for PDT to treat burns infected with multidrug-resistant A. baumannii in vivo.