Phototherapy Regimen and Different Substrates Interfere on Candida Albicans Biofilms

Objectives: The aim of this study was to compare the effects of different regimens of photodynamic antimicrobial chemotherapy (PACT) on Candida albicans biofilms formed in two substrates.
Methods: The outcomes of PACT were compared between biofilms formed on the bottom of 24-well polystyrene plates and on acrylic resin samples [1.0x1.0x0.5 cm; Ra=0.3±0.03 μm]. PACT was performed on C. albicans SN425 biofilms cultivated on RPMI medium. Biofilms were incubated for 5 min with the photosensitizer toluidine blue O (100 μM) before red light [660 nm; 175.5 J/cm²] application for 2 min. This treatment was performed either once after 48-h biofilm formation or twice-daily on the course of 48-h. Biofilms were then evaluated for colony-forming units (CFU) and dry-weight (DW). As a control, non-treated biofilms were formed on both substrates.
Results: CFU and DW of non-treated biofilms formed on 24-well plates (6.51±0.12 Log₁₀ CFU/mL; 7.4±0.8 mg) are similar to those formed on acrylic resins (6.27±0.03 Log₁₀ CFU/mL; 8.8±1.2 mg). However, one time PACT application on biofilms formed on 24-well plates did not reduce the CFU or the DW (6.06±0.24 Log₁₀ CFU/mL; 10.5±0.9 mg). In contrast, when PACT was applied twice-daily to such biofilms, reductions of 2.25 Log₁₀ and 3.3 mg of DW were observed. Biofilms formed on acrylic resin showed to be more susceptible to PACT either when applied one time or twice-daily, showing a reduction of 3 Log₁₀ on both conditions. Nevertheless, on acrylic resins, PACT did not reduce the DW when applied one time (12.6±5.2 mg), but when applied twice-daily, PACT reduced 6 mg of the DW compared to the non-treated biofilms.
Conclusions: Briefly, biofilms formed on the bottom of 24-well polystyrene plates are more resistant to PACT than biofilms formed on acrylic resins. Moreover, applying PACT twice-daily is a better regimen, once it reduces C. albicans biofilm development.