Inactivation of planktonic cells and inhibitory effect on post-treatment biofilm formation of methicillin-resistant Staphylococcus aureus by photodynamic treatment with IR780 iodide loaded mesoporous silica nanoparticles and near infrared light

The rapid spread of antimicrobial resistance is one of the biggest threats to global health. In the search for new treatment approaches that can eradicate pathogens without inducing drug-resistant strains, photodynamic therapy stands as a promising one. The aim of this study was to investigate the antimicrobial photodynamic potential of mesoporous silica nanoparticles (MSN) loaded with IR780 iodide on one of the most common multidrug-resistant bacteria both in hospitals and in the community, which is methicillin-resistant Staphylococcus aureus (MRSA). Mesoporous silica nanoparticles loaded with IR780 iodide were synthesized, their photodynamic and photothermal properties were examined, and their antimicrobial photodynamic potential against one methicillin-susceptible Staphylococcus aureus (MSSA), and one MRSA strain was investigated. Irradiation was achieved via a 785 nm diode laser (500 mW/cm2, 5 min). Viable bacterial cells were counted by serial dilution method. The post-treatment biofilm recurring ability of MRSA was assessed 24 h post-PDT treatment using Crystal Violet assay. Scanning Electron Microscopy (SEM) of post-treatment biofilms was acquired. Data were analyzed by ANOVA followed by Tukey's test (p ≤ 0.05). Results revealed that mesoporous silica nanoparticles loaded with IR780 iodide-mediated photodynamic therapy were effective in killing both tested strains. The antimicrobial effect was stronger on MRSA, in which 99.97% of photokilling (3.54 log reduction) was observed. The killing was mainly due to the photodynamic action of the nanoparticles. Post-treatment biofilm recurring ability of MRSA was much less in the treated group than that of the control group (50% inhibition), as confirmed by both optical density at 570 nm (OD570) measurement, and Scanning Electron Microscope (SEM) imaging.