Microorganism-Dependent Action of Antimicrobial Photodynamic Therapy

Objectives: This study aims to evaluate the efficacy of Antimicrobial Photodynamic Therapy (aPDT) in reducing the viability of biofilms of Streptococcus mutans UA159 and Enterococcus faecalis ER5/1.
Methods: Microorganisms were cultured in a modified semi defined medium (SDMY) plus 0.4% glucose and grown on polystyrene pegs in a high-throughput biofilm model for 24 h. aPDT was evaluated using different concentrations of a hematoporphyrin derivative (0, 0.5, 1.0 or 2.0 mg.mL^-1 Photogem^®) combined with different light doses of visible red LEDs (0, 3.32 or 4.43 J.cm^-2 Biotable^®, 630 nm, 50 mW.cm^-2). The metabolic activity of bacterial cells was determined by resazurin assay at 485 nm-excitation and 580 nm-emission wavelengths.
Results: A light and photosensitizer dose-dependent reduction of metabolic activity of S. mutans and E. faecalis biofilms was observed. S. mutans biofilms treated with either Biotable^® or Photogem^® alone displayed similar metabolic activity to the treatment group. Otherwise, E. faecalis biofilms treated with 4.43 J.cm^-2 light dose alone showed a metabolic reduction of 31.8%. Therefore, E. faecalis ER5/1 biofilms were more light-sensitive than S. mutans UA159 biofilms.
Conclusions: aPDT was able to decrease the metabolic activity of S. mutans UA159 and E. faecalis ER5/1 biofilms. The efficacy of aPDT in reducing the metabolic activity of biofilms was microorganism-dependent.