Antifungal Susceptibility Testing and Clinical Efficacy Observation of Methylene Blue Photodynamic Therapy in Treating Trichophyton indotineae Infections

Abstract Purpose: Superficial fungal infections caused by Trichophyton indotineae are spreading globally, and mutations in its SQLE gene (F397L/L393S) are leading to increasing resistance to traditional antifungal drugs such as terbinafine. ¹ There is an urgent need to explore new treatment options. Photodynamic therapy (MB-PDT) utilizes methylene blue-mediated reactive oxygen species (ROS) to disrupt fungal mitochondrial function, demonstrating broad-spectrum potential against drug-resistant strains. This study aims to evaluate the clinical efficacy of methylene blue photodynamic therapy (MB-PDT) for infections caused by Trichophyton mentagrophytes. Patients and methods：The minimum inhibitory concentration (MIC) of methylene blue and terbinafine was determined for clinical isolates through in vitro antimicrobial susceptibility testing. The donor for this isolate was a 25-year-old Indian male patient who underwent methylene blue photodynamic therapy. The patient presented with widespread, multiple skin lesions on the extremities, abdomen, and inguinal region, which were confirmed through microscopic examination and ITS sequencing as a multidrug-resistant Trichophyton mentagrophytes infection. Strains were collected from different body regions (buttocks, upper limbs, lower limbs, and trunk) for in vitro antimicrobial susceptibility testing. The patient was then treated using a randomized controlled design: the experimental group (forearm lesions) received topical application of 10% methylene blue combined with 630 nm red light irradiation (100 J/cm²), once weekly for 20 minutes, for a total of 4 weeks; the control group (remaining lesions) received only topical application of 1% terbinafine cream twice daily. Result： In vitro antimicrobial susceptibility testing revealed that the patient's Trichophyton mentagrophytes strain was more sensitive to methylene blue (MIC = 1.0 μg/mL) and exhibited strong resistance to terbinafine (MIC = 8.0 μg/mL). After two weeks of treatment, the experimental group showed resolution of scales and negative fungal microscopy (hyphae density < 1/HPF), while the control group still had residual hyphae (> 5/HPF). By the fourth week, the experimental group had negative fungal microscopy results, whereas the control group still had residual hyphae, with microscopy results remaining positive, necessitating further extension of the treatment duration. Conclusion：Methylene blue photodynamic therapy (PDT) can rapidly clear drug-resistant Trichophyton mentagrophytes infections. It is more effective than traditional topical antifungal drugs and provides a safe and effective alternative treatment for drug-resistant dermatophytosis.