Antimicrobial Activity and Mechanism of Action of Silver Nanoparticles Synthesized from Leaf Extracts of Ocimum gratissimum Against MDR Clinical Isolates in Owerri, Imo State

The global antimicrobial resistance crisis has accelerated interest in nanotechnology-based antimicrobial agents, particularly biogenically synthesized silver nanoparticles (AgNPs) that leverage both metallic and phytochemical antimicrobial mechanisms. This study synthesized AgNPs from aqueous leaf extract of Ocimum gratissimum (scent leaf), a plant widely used in Nigerian ethnomedicine, and evaluated their antimicrobial activity and mechanism of action against multidrug-resistant clinical isolates obtained from Imo State University Teaching Hospital, Owerri, South East Nigeria. AgNP synthesis was confirmed by UV-Vis spectrophotometry (maximum absorption at 420 nm), dynamic light scattering for particle size characterization (mean: 28.4 nm), and transmission electron microscopy. Antimicrobial activity was tested against MDR E. coli, MRSA, K. pneumoniae, and Candida albicans by agar diffusion and broth microdilution. Mechanistic studies examined membrane permeabilization by propidium iodide uptake assay and reactive oxygen species (ROS) generation by DCF fluorescence assay. Results showed significant antimicrobial activity (MIC: 3.12 to 12.5 microg/mL) against all test organisms including isolates resistant to 8 or more antibiotics. Mechanistic assays confirmed dose-dependent membrane permeabilization and elevated ROS generation. AgNPs demonstrated synergistic activity when combined with ciprofloxacin against MRSA. These findings establish O. gratissimum-derived AgNPs as promising nano-antimicrobial candidates warranting further cytotoxicity and in vivo investigation. Keywords: silver nanoparticles, Ocimum gratissimum, antimicrobial resistance, MRSA, nanoparticle synthesis.