Effect of methylene blue-mediated antimicrobial photodynamic therapy on dentin caries microcosms

Antimicrobial photodynamic therapy (aPDT) has been proposed as an adjuvant treatment of dental caries, although there are no well-defined protocols to its clinical application. This study aimed to evaluate the influence of aPDT on the viability of microorganisms, vitality of biofilms, and lactic acid production of dentin caries microcosms. Biofilms were grown on bovine dentin discs in anaerobic conditions at 37 °C for 5 days, inoculating infected carious dentin in modified McBain medium plus 1% sucrose. The biofilms were treated by the combination of deionized water or 100 mg L^-1 methylene blue (MB) with 0, 37.5, or 75 J cm^-2 LED at 630 nm. The counts of total microorganisms, total streptococci, mutans streptococci, and total lactobacilli were determined by colony-forming units (CFU). The vitality of microbial cells in intact biofilms was measured by confocal laser scanning microscope (CLSM). The lactic acid production was analyzed by enzymatic spectrophotometry at 340 nm. Statistical analysis was conducted by Kruskal-Wallis and post hoc Dunn's tests (P < 0.05). MB and 37.5 J cm^-2 LED alone did not interfere in the viability of microorganisms, unlike 75 J cm^-2 LED alone that decreased the total microorganism and lactobacillus counts. The combination of MB and 75 J cm^-2 LED reduced the viability of all microorganisms and the vitality of intact biofilms. The production of lactic acid was statistically lower in all treatment groups in comparison with that of the control (no treatment), except for MB alone. Therefore, the MB-mediated aPDT was effective in controlling the viability, vitality and the acidogenicity of dentin caries microcosms.