Susceptibility of Superoxide Dismutase-Deficient Candida Albicans to Antimicrobial Photodynamic Therapy

Objectives: Given that superoxide dismutase enzymes confer resistance to oxidative stress, this study aimed to assess the effectiveness of antimicrobial photodynamic therapy (aPDT) using methylene blue as the photosensitizer and red light on two Candida albicans (C. albicans) strains: a wild-type strain and a superoxide dismutase (SOD)-deficient mutant strain.
Methods: Biofilm cultures of both C. albicans strains (wild-type, SC5314, and SOD-deficient, CA-IF070 sod4Δ/Δ sod5Δ/Δ sod6Δ/Δ) were cultured aerobically for 24 hours at 37°C in light-blocking 96-well polystyrene plates. Each strain was divided into four groups: microorganism only (control; no light and no dye), light control (microorganism + light but no dye), dye control (microorganism + dye but no light), and experimental group (microorganism + dye + light). Sterile methylene blue (MW 373.89 g/mol, 95% pure) was added to the wells to achieve a final concentration of 50 µM. Samples were incubated in the dark with the dye for 5 minutes. After dye removal, the samples were irradiated with a red laser probe from a Thor laser system (600 nm, 73 J/cm², 70 mW, 14.5 mm distance to the biofilm) for 5 minutes. The wells were washed twice with sterile PBS, serially diluted, and plated onto Sabouraud agar plates for 24 hours to count colony-forming units (CFUs). Statistical analysis was performed using two-way ANOVA/Tukey (α=0.05).
Results: All treated groups exhibited a slight reduction in CFU counts compared to the untreated controls for both the wild-type and SOD-deficient strains, indicating the photodynamic activity of methylene blue under red light. However, the comparisons showed no significant difference between the wild-type and SOD-deficient strains (p= 0.99) or within the intragroup analysis (p=0.99).
Conclusions: These findings suggest that the SOD-deficient strain does not exhibit increased susceptibility to aPDT under the conditions tested. Future studies should investigate alternative dosimetries or other approaches to enhance aPDT efficacy against C. albicans biofilms.