摘要： Aim: To evaluate the in vitro antimicrobial effects of photodynamic therapy (PDT). Methods: The microorganism indicators were: Candida albicans, Pseudomonas aeruginosa,Enterococcus faecalis and Staphylococcus aureus. A microbial pool was prepared (108 cells/mL), from which aliquots were transferred to culture plates for carrying out the PDT using methylene blue (50 μM) and low-power laser (660 nm, 100 mW and 9 J).The effect of methylene blue alone, low power laser and the absence of treatments were evaluated. Then, aliquots of 1 μL were plated in a media culture, the number of colony forming units (CFU/mL) was obtained and the data submitted to the F test (ANOVA) with Tamhane’s comparisons. Results:The laser radiation in the presence of methylene blue was able to eliminate 74.90% of C. albicans, 72.41% of P. aeruginosa, 96.44% of E. faecalis and 95.42% of S. aureus, thus statistically significant differences were found among the groups (p<0.001). Conclusions: PDT was effective in reducing the number of viable cells in the studiedmicroorganisms, especially E. faecalis and S. aureus.