Effectiveness of antimicrobial photodynamic therapy (AmPDT) on Staphylococcus aureus using phenothiazine compound with red laser

The aim of this study was to evaluate, in vitro, the bactericidal effect of antimicrobial photodynamic therapy (AmPDT) using phenothiazinium dyes (Toluidine Blue O and methylene blue, 1:1) using different concentrations (100, 50, 25, 12.5, and 6.25 μg/mL) associated to red laser with different energy densities (2.4, 4.8, 7.2, 9.6, and 12 J/cm²) on a strain of Staphylococcus aureus (ATCC 23529). On this study, tests were performed in triplicate and the samples were distributed into 36 test groups: Control and bacterial suspensions were irradiated with the different energy densities, respectively, in the absence of photosensitizer, bacterial suspensions were irradiated with the laser in the different concentrations of the photosensitizer, and finally bacterial suspensions only in the presence of phenothiazinium dye. The pre-irradiation time was 5 min. Therefore, we analyzed the potential of the AmPDT by counting colony-forming units. The logarithm of CFU/mL (log10 CFU/mL) was calculated and the data was analyzed statistically (ANOVA, Tukey’s test, p < 0.05). The results showed that the association 50 and 100 μg/mL with 12 J/cm² showed the highest percentage of inhibition (100 %). Based upon the present results, it may be concluded that the AmPDT was able to enhance the antimicrobial effect of phenothiazines and both concentration of the compound and energy density are important factors for greater effectiveness of therapy.