Antimicrobial Blue Light Inactivation of Gram-Negative Pathogens in Biofilms: In Vitro and in Vivo Studies

Background. Biofilms affect >80% bacterial infections in human and are usually difficult to eradicate because of their inherent drug resistance. Methods. We investigated the effectiveness of antimicrobial blue light (aBL) (wavelength, 415 nm) for inactivating Acinetobacter baumannii or Pseudomonas aeruginosa biofilms in 96-well microplates or infected mouse burn wounds. Results. In vitro, in 96-well microplates, exposure of 24-hour-old and 72-hour-old A. baumannii biofilms to 432 J/cm² aBL resulted in inactivation of 3.59 log₁₀ and 3.18 log₁₀ colony-forming units (CFU), respectively. For P. aeruginosa biofilms, similar levels of inactivation - 3.02 log₁₀ and 3.12 log₁₀ CFU, respectively - were achieved. In mouse burn wounds infected with 5 × 10⁶ CFU of A. baumannii, approximately 360 J/cm² and 540 J/cm² aBL was required to inactivate 3 log₁₀ CFU in biofilms when delivered 24 and 48 hours, respectively, after bacterial inoculation. High-performance liquid chromatography analysis revealed the presence of endogenous porphyrins in both A. baumannii and P. aeruginosa. TUNEL assay detected no apoptotic cells in aBL-irradiated mouse skin at up to 24 hours after aBL exposure (540 J/cm²). Conclusions. aBL has antimicrobial activity in biofilms of A. baumannii and P. aeruginosa and is a potential therapeutic approach for biofilm-related infections.