Bacteria-specific phototoxic reactions triggered by blue light and phytochemical carvacrol

Development of alternatives to antibiotics is one of the top priorities in the battle against multidrug-resistant (MDR) bacterial infections. Here, we report that two naturally occurring nonantibiotic modalities, blue light and phytochemical carvacrol, synergistically kill an array of bacteria including their planktonic forms, mature biofilms, and persisters, irrespective of their antibiotic susceptibility. Combination but not single treatment completely or substantially cured acute and established biofilm-associated Acinetobacter baumannii and methicillin-resistant Staphylococcus aureus infections of full thickness murine third-degree burn wounds and rescued mice from lethal Pseudomonas aeruginosa skin wound infections. The combined therapy diminished bacterial colony-forming units as high as 7.5 log₁₀ within 30 min and introduced few adverse events in the survival of cocultured mammalian cells, wound healing, or host DNA. Mechanistic studies revealed that carvacrol was photocatalytically oxidized into a series of photoreactive substrates that underwent photolysis or additional photosensitization reactions in response to the same blue light, forming two autoxidation cycles that interacted with each other resulting in robust generation of cytotoxic reactive oxygen species. This phototoxic reaction took place exclusively in bacteria, initiated by blue light excitation of endogenous porphyrin-like molecules abundantly produced in bacteria compared with mammalian cells. Moreover, no bacterial resistance developed to the combined treatment after 20 successive passages. This highly selective phototoxic reaction confers a unique strategy to combat the growing threat of MDR bacteria.