Enhancing Antimicrobial Photodynamic Therapy: The Role of Reserpine in Inhibiting Efflux Pumps with Silver Nanoparticles and Methylene Blue*

Efflux pumps are critical active transport systems used by cells to expel toxic substances, including antibiotics and photosensitizers, contributing to antimicrobial resistance. Efflux pump inhibitors (EPIs) play a key role in enhancing antimicrobial therapies. This study examines the effects of the EPI reserpine on the photodeactivation rate of pathogens when combined with silver nanoparticles (Ag NPs) and methylene blue (MB). The combination of reserpine with Ag NPs and MB resulted in a significantly higher pathogen deactivation rate compared to Ag NPs and MB alone, indicating that reserpine disrupts cellular mechanisms that typically expel toxic agents. Additionally, different sizes of Ag NPs were tested, showing that reserpine consistently improves deactivation, regardless of nanoparticle size. Molecular docking revealed that reserpine has a higher affinity for the AcrB protein, a component of the AcrAB-TolC efflux pump, compared to MB. The enhanced deactivation is due to reserpine blocking the efflux pump, retaining MB within the cells.