Effects of photodynamic therapy on rapidly growing nontuberculous mycobacteria keratitis

Purpose. The authors investigated the antimicrobial effect of methylene blue (MB)-mediated photodynamic therapy (PDT) on Mycobacterium fortuitum keratitis. Methods. In the in vitro study, the mycobacterial suspension and colonies were treated with the following: no MB, no light (normal control); MB and no light (dye control); light and no MB (light control); MB and light (PDT). Morphologic characteristics were examined by transmission electron microscopy. The bactericidal effects of combined PDT and antibiotic therapy (ciprofloxacin, moxifloxacin, and amikacin) were determined using the broth microdilution technique. Twenty-one rabbits with Mycobacterium keratitis were randomly divided into three groups (no treatment, topical amikacin treatment, and PDT combined with amikacin treatment). The clinical features of keratitis were scored and graded before treatment and before euthanatization. The diseased corneas were trephined for quantitative bacteriologic analysis to determine the antibacterial efficacy of the treatment. Results. In the in vitro tests, the bacterial count had a 2-log reduction immediately after PDT treatment at 100 J/cm² with 10^-3% MB. After PDT at 100 J/cm² with 10^-2% MB, almost no viable bacteria were detected. PDT had a synergistic antimicrobial effect in combination with antibiotics. The phototoxicity occurred in the cytoplasm first and then disrupted the mycobacterial cell walls by lysis. In the rabbit keratitis model, combined PDT resulted in significantly less bacterial burden (P < 0.01) than in the amikacin group.Conclusions. This study demonstrated the effectiveness of MB-mediated PDT against Mycobacterium fortuitum. PDT could be a potential alternative treatment for nontuberculous mycobacterial corneal infections.