Antimicrobial photodynamic therapy fighting polymicrobial infections - a journey fromin vitrotoin vivo

Rapidly evolving multidrug resistance renders conventional antimicrobial strategies increasingly inefficient. This urges the exploration of alternative strategies with a lower potential of resistance development to control microbial infections. A promising option is antimicrobial photodynamic therapy (aPDT), especially in the setting of wound infections. In this study its effectiveness was tested as a treatment option for polymicrobially infected wounds in bothin vitroandin vivomodels. First, aPDT was applied to wound-relevant Gram-positive and Gram-negative bacteria in planktonic culture as the standardin vitrotest system and compared different media to show a possible dependency of the therapy on the surrounding environment. In a second step, aPDT was investigated in anin vitromodel mimicking the wound bed conditions using fibrin-coated culture plates. Finally, we tested aPDTin vivoin a polymicrobial infected wound healing model in immunocompromised BALB/c mice.In vitro, it was shown that the bactericidal effectiveness of aPDT was strongly dependent on the surrounding environment of the phototoxic reaction.In vivo, the significant delay in wound healing induced by polymicrobial infection was drastically diminished by a two-times application of aPDT using 100 mu M methylene blue (generally regarded as safe for topical application on human skin) and 24 J cm(-2)pulsed red LED light. Our experiments suggest that aPDT is capable of significantly improving wound healing also in complicated polymicrobially infected wound situations.