The use of polymeric nanoparticles in antimicrobial photodynamic therapy

Objectives: To study the effects of poly(lactic-co-glycolic acid) (PLGA) nanoparticles loaded with the photosensitizer methylene blue (MB) and light against Enterococcus faecalis in vitro. Methods: MB-loaded PLGA nanoparticles were prepared and characterized. The uptake and distribution in E. faecalis was investigated by transmission electron microscopy (TEM) after incubation with PLGA complexed with colloidal gold particles for 2.5, 5 and 10 min. For the photodynamic treatment of microorganisms, aliquots of bacterial suspensions were incubated with MB-loaded PLGA nanoparticles (final concentration: 6.25 mg/ml equivalent to MB) for 10 min followed by exposure to red light at 665 nm with an irradiance of 100 mW/cm² and an energy fluence of 60 J/cm². After photodynamic therapy, survival fractions were calculated by counting colony-forming units. Results: A large fraction (>99%) of nanoparticles were found to be concentrated onto the cell walls of microorganisms at all three time points. Nanoparticles loaded with MB reduced bacterial survival by 34% in the absence of light, whereas PDT produced approximately 2 log₁₀ bacterial killing. Conclusions: The data suggest that PLGA nanoparticles encapsulated with photoactive drugs may have an application in antimicrobial photodynamic therapy. Supported by NIDCR (RO1-DE-16922).