Different susceptibility of spores and hyphae of Trichophyton rubrum to methylene blue mediated photodynamic treatment in vitro

Background In recent years, methylene blue mediated‐photodynamic therapy (MB‐PDT) has proved to be an effective inhibitor to a variety of microorganisms, including Trichophyton rubrum , the most common dermatophyte worldwide. However, previous studies mainly focused on the spore form of T rubrum , but rarely on its hyphal form, although the latter is the main pathogenic form of T rubrum in vivo. Objective To investigate the inhibitory effect of MB‐PDT on T rubrum in different growth phases in vitro. Methods The suspensions of spores and hyphae obtained from T rubrum (ATCC28188) were prepared, respectively, incubated with MB solution (0.15‐40 μg/mL) and irradiated with 635 nm red light. Varied light energy and MB concentration were used. The specimen in the absence of light exposure or/and MB served as controls. MIC determination, colony counts and MTT assay were employed to evaluate the antifungal effect of MB‐PDT. Results The MICs of MB‐PDT for hyphae and spores of T. rubrum were 6.300 ± 1.072 μg/mL and 1.984 ± 1.072 μg/mL, respectively, at a fixed light dose of 60 J/cm 2 . CFU counts gave the minimum critical combinations of MB concentration and light dose to achieve 100% inhibitory rate. For hyphae, they were 5 μg/mL + 100 J/cm 2 or 10 μg/mL + 60 J/cm 2 . For spores, they were 1.25 μg/mL + 40 J/cm 2 or 5 μg/mL + 20 J/cm 2 . The outcomes of MTT assay were consistent with those of CFU counts, but less accurate. Conclusion MB‐PDT is a potent inhibitor to both spores and hyphae of T. rubrum in vitro, and the spores are more sensitive to it. Its antifungal efficacy is positively correlated with the concentration of MB and light dose.