Highly Efficient Photodynamic Inactivation of Multidrug-resistant Pseudomonas aeruginosa, Staphylococcus aureus and Acinetobacter baumannii Strains by Toluidine Blue-graphene Oxide Quantum Dot/Ag Nanoparticle Conjugate.

Photodynamic inactivation (APDI) is a novel and promising approach for the treatment of bacterial infections. Nanoparticle-based photosensitizer delivery systems can improve the efficacy of APDI. This study aimed to determine whether graphene quantum dot/silver nanoparticles (GQD/AgNPs) hybrid as a nanocarrier system could enhance the efficacy of toluidine blue (TBO)-mediated PDI against multidrug-resistant (MDR) Pseudomonas aeruginosa, Staphylococcus aureus and Acinetobacter baumannii strains. At first, the negatively-charged GQD/AgNP hybrids were synthesized using the pyrolysis method combined with hydrothermal processing. Then, the cationic TBO was immobilized to anionic GQD/AgNP hybrids via electrostatic attraction. The tested bacterial strains growing as planktonic cells were treated with TBO alone and TBO-conjugated GQD/AgNPs nanohybrid and subsequently exposed to laser light (650 nm). Meanwhile, the impacts of PDI using TBO-conjugated GQD/AgNP hybrids on planktonic bacterial cells were visualized using field emission scanning electron microscopy (FESEM). Also, the inhibitory effect of TBO-GQD/AgNPs-mediated PDI on biofilm formation was assessed. PDI using TBO-GQD/AgNPs conjugate revealed significant antibacterial activity compared to TBO-mediated PDI, so that the planktonic cells of P. aeruginosa/A. baumannii and S. aureus were almost completely eradicated by TBO-GQD/AgNPs-mediated PDI at Sub-lethal concentrations of 10 and 5 µM of TBO, respectively. FESEM analysis confirmed the potent ability of TBO-GQD/AgNPs-mediated PDI to kill P. aeruginosa, S. aureus and A. baumannii cells. Also, the results indicated that PDI mediated by TBO-GQDs/AgNP (at sub-lethal dose) completely inhibited biofilm formation by all three tested bacteria. In summary, PDI mediated by TBO-GQD/AgNPs offer a new modality for fast and efficient elimination of MDR P. aeruginosa, S. aureus and A. baumannii strains.