Antimicrobial photodynamic therapy fighting polymicrobial infections – a journey from in vitro to in 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 both in vitro and in vivo models. First, aPDT was applied to wound-relevant Gram-positive and Gram-negative bacteria in planktonic culture as the standard in vitro test 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 an in vitro model mimicking the wound bed conditions using fibrin-coated culture plates. Finally, we tested aPDT in vivo in 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 μM 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.