Candida biofilm matrix as a resistance mechanism against photodynamic therapy

Background: Antimicrobial photodynamic therapy (aPDT) efficiency on Candida albicans is recognized in free-floating cultures. Though, the lack of aPDT effectiveness against C. albicans organized in biofilms is still unclear. This study aimed to explore the role of the extracellular matrix (ECM) in the protection against aPDT in C. albicans biofilms. Methods: C. albicans SN 425 wild-type and two mutant strains CNJ 2302; Δ/Δefg1 and CJN 2330; Δ/Δtec1 (ECM deficient) were used. Biofilms were grown on 24-well plates and exposed twice-daily to aPDT with 44 μM toluidine blue-O (TBO) for 5 min followed by red light (635 nm) for 1 min (87.6 J/cm²) or 2 min (175.2 J/cm²). Application of just TBO, light, 0.12% chlorhexidine, and ultrapure water were used as controls. After 48 h, biofilms were assessed for dry-weight (DW), colony forming units (CFU), extracellular DNA (eDNA), soluble and insoluble protein (SP/IP), water-insoluble (alkali-soluble) polysaccharide (ASP), water-soluble polysaccharides (WSP), and confocal scanning laser microscopy. Results: The strains with ECM deficient were affected by aPDT. For the mutant strain Δ/Δefg1, aPDT significantly reduced CFU, ASP, DW, eDNA, WSP and IP when compared to NC (p<0.001) and for the Δ/Δtec1, aPDT significantly reduced CFU, eDNA, IP and SP. Whereas CFU, DW, ASP of the wild-type strain biofilms were not reduced (p>0.05). Conclusions: C. albicans strains with reduced ECM compounds were more sensitive to aPDT suggesting that the ECM may have a significant protection role from aPDT in C. albicans biofilms.