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Bacteria often colonize, and sometimes infect, open wounds. Antimicrobial resistance may complicate measures to decrease bacterial load or eliminate the infectious organisms. Photodynamic therapy (PDT) using a photosensitizing drug and visible light has been used to kill bacteria, viruses, and fungi. Most photosensitizers show photodynamic activity against Gram-positive organisms, but for Gram-negative bacteria, cationic photosensitizers are needed. Phenothiazinium derivatives are cationic photosensitizers that absorb light in the 630–680-nm spectrum. One of these, designated PPA904, is under development for clinical use. Investigators including employees of the manufacturer conducted a clinical study of PPA904 as antimicrobial PDT for chro…