Methylene blue-loaded polymeric nanoparticles: Effect of serum on bacterial photodestruction

Objectives: To investigate the effect of serum proteins on the photodynamic activity of biodegradable and biocompatible polymeric nanoparticles loaded with the photosensitizer methylene blue (MB) against Enterococcus faecalis. Methods: (MB)-encapsulated nanoparticles of poly(lactic-co-glycolic acid) (PLGA) with negative and positive surface charge were prepared and characterized. E. faecalis species were sensitized in planktonic phase with MB-loaded PLGA nanoparticles (final concentration: 12.5 ìg/ml equivalent to MB) for 15 minutes followed by exposure to red light at 665 nm with an irradiance of 100 mW/cm2 and energy fluence of 30 J/cm2 in the presence of brain heart infusion (BHI) broth or phosphate buffered saline (PBS). After photodynamic therapy, all samples were serially diluted, cultured on blood agar plates and survival fractions were calculated by counting colony-forming units (CFU). Results: In the presence of light, the cationic MB-loaded nanoparticles produced 80% and 90% bacterial killing when dissolved in BHI broth and PBS, respectively. The anionic MB-loaded nanoparticles eliminated microorganisms by 13% in BHI broth and 72% in PBS. Conclusions: The photodynamic effects of MB-loaded PLGA nanoparticles on E. faecalic species were probably reduced by the presence of serum proteins in BHI broth. Supported by NIDCR grant RO1-DE-16922.