Synergistic antimicrobial effect of photodynamic therapy and ciprofloxacin

The occurrence of a variety of pathogens resistant to current antibiotics remains the major problem in medical care, especially when bacterial infections are established as biofilms. In this study, we propose the use of photodynamic therapy (PDT) as a monotherapy and associated with antibiotic as an alternative treatment. The aim of this study was to analyze the effects of PDT mediated by methylene blue (MB) on Staphylococcus aureus (ATCC 25923) and Escherichia coli (ATCC 25922) in both biofilm and planktonic phases. Several concentrations of MB and light doses were tested. The bactericidal effects of PDT as a monotherapy did not increase with the concentration of photosensitizer, but were light dose-dependent. In addition, bacteria in biofilms were less affected than cells in the planktonic phase. Although not concentration-dependent, the disruption effect of PDT on biofilms was clearly illustrated by scanning electron microscopy (SEM). We also carried out experiments that evaluated the synergistic effect of photodynamic therapy and the antibiotic ciprofloxacin. The best results were obtained after combination treatment of photodynamic therapy followed by ciprofloxacin on biofilms, which increased bacterial reduction on biofilms, resulting in a 5.4 log reduction for S. aureus biofilm and approximately 7 log for E. coli biofilm.