Alireza Nouhi, Khosro Issazadeh, Alireza Habibi, Leila Asadpour, Saeid Zarrabi
Abstract Biofilm formation and quorum sensing (QS) systems are key determinants of pathogenicity and therapeutic resistance in multidrug-resistant (MDR) Pseudomonas aeruginosa , making these pathways attractive targets for combating chronic biofilm-associated infections. In this study, green-synthesized silver nanoparticles derived from Capparis spinosa fruit extract were functionalized with quercetin (AgNPs@Qu), and their antibiofilm and QS-modulatory activities were evaluated against MDR clinical isolates of Pseudomonas aeruginosa . Physicochemical characterization by UV–Vis spectroscopy, FTIR, XRD, zeta potential analysis, FE-SEM, DLS, and ICP-OES confirmed the successful formation of nanoparticles with a face-centered cubic (fcc) crystalline structure, an average diameter of approximately 30 nm, and a quercetin surface coating. Compared with unfunctionalized AgNPs, AgNPs@Qu exhibited a 4- to 8-fold reduction in minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) against MDR isolates and significantly suppressed bacterial growth. More importantly, AgNPs@Qu displayed potent antibiofilm activity, reducing biofilm biomass by more than 70–85% in hyper-biofilm-forming isolates, a performance comparable to that of ciprofloxacin. Furthermore, AgNPs@Qu exerted a concentration-dependent, biphasic effect on pyocyanin production, characterized by transient stimulation at low sub-inhibitory concentrations and relative inhibition at higher concentrations, reflecting a modulatory action of quercetin on quorum sensing pathways. These findings indicate that the antibiofilm and QS-modulatory properties of the AgNPs@Qu nanocomposite, beyond its bactericidal action, contribute to its enhanced in vitro efficacy compared with unfunctionalized AgNPs against MDR Pseudomonas aeruginosa isolates, positioning it as a promising candidate for further preclinical investigation as an antivirulence-based strategy.