Candida Biofilm Matrix as a Resistance Mechanism Against Photodynamic Therapy

Objectives: The effects of antimicrobial photodynamic therapy (aPDT) on Candida albicans have been shown in several studies. However, microorganisms in biofilms are less susceptible to aPDT than their planktonic counterparts, and one of the potential mechanisms could be the fact that extracellular matrix (ECM) limits penetration of the photosensitizers and light. To test the hypothesis that the ECM protects biofilms, the aim of this study was to compare the outcomes of aPDT on biofilms of C. albicans wild-type strain (WT, SN425) and mutant strain with reduced ECM components (Δ/Δ efg1, CJN2302).
Methods: Biofilms treated with aPDT were first incubated for 5 min with the photosensitizer toluidine blue O (TBO) at the concentration of 44 μM before red light (570-690 nm) application for 1 min (87.6 J/cm²) or 2 min (175.5 J/cm²). As negative control (NC), biofilms were treated with sterile ultrapure water for 1 min. As positive control (PC), 0.12% chlorhexidine was applied for 1 min. The effect of TBO alone and light alone for 1 min or 2 min were also assessed. These treatments were performed twice-daily on the course of 48-h. Biofilms were evaluated for colony-forming units (CFU). Data was analyzed by Kruskall-Wallis and Mann–Whitney tests with Bonferroni correction (α = 0.05).
Results: aPDT did not reduce CFU of the WT strain biofilms, approaching NC levels (p > 0.282). However, for the mutant strain Δ/Δ efg1, aPDT application both for 1 and 2 min significantly reduced the CFU compared to the NC (p < 0.001), approaching PC levels (p > 0.125).
Conclusions: Therefore, the extracellular matrix may have a key role on the protection of C. albicans biofilms from aPDT.