Tehran University of Medical Sciences Nanomedicine Research Journal 2476-3489 11 2 2026 06 01 Eco-Friendly Extracellular Biosynthesis of Silver Nanoparticles Using a Novel Bacillus Strain: Characterization and Potent Antibacterial Activity Against Multidrug-Resistant Pathogens 199 209 734215 10.22034/nmrj.2026.02.010 EN Mohammadali Nahofte Cellular and Molecular Biology Research Center, Health Research Institute, Babol University of Medical Sciences, Babol, Iran Fatemeh Tohidi Cellular and Molecular Biology Research Center, Health Research Institute, Babol University of Medical Sciences, Babol, Iran Department of Medical Microbiology and Medical Biotechnology, Faculty of Medicine, Babol University of Medical Sciences, Babol, Iran Abazar Pournajaf Department of Medical Microbiology and Medical Biotechnology, Faculty of Medicine, Babol University of Medical Sciences, Babol, Iran Infectious Diseases and Tropical Medicine Research Center, Health ResearchInstitute, Babol University of Medical Sciences, Babol, Iran Mehdi Rajabnia Department of Medical Microbiology and Medical Biotechnology, Faculty of Medicine, Babol University of Medical Sciences, Babol, Iran Infectious Diseases and Tropical Medicine Research Center, Health ResearchInstitute, Babol University of Medical Sciences, Babol, Iran Journal Article 2025 07 12 <strong>Objective(s): </strong>Biosynthesized nanoparticles have attracted significant attention in biomedical research due to their biocompatibility, natural origin, and potent antibacterial properties. Silver nanoparticles (AgNPs), in particular, are valued for their cost-effectiveness, eco-friendliness, and strong antimicrobial activity. This study aimed to establish a sustainable and scalable extracellular green synthesis platform for AgNPs using Bacillus strains isolated from spring water.<br><strong>Methods:</strong> Eighteen Bacillus isolates were screened for their ability to reduce silver ions. The most efficient strain-mediated synthesis was optimized under 3 mM AgNO₃, pH 10, 37 °C, for 72 h. The synthesized AgNPs were characterized using UV–visible spectroscopy, scanning electron microscopy (SEM), dynamic light scattering (DLS), X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) analyses to assess morphology, size, crystallinity, surface charge, and biomolecular capping.<br><strong>Results:</strong> UV–Vis spectroscopy revealed a distinct surface plasmon resonance peak at 430 nm. SEM and DLS analyses confirmed well-dispersed, nearly spherical nanoparticles with diameters of 30–50 nm and high colloidal stability (zeta potential −34/9 mV). XRD patterns indicated a crystalline face-centered cubic structure, while FTIR spectra suggested biomolecular capping via amide and hydroxyl functional groups. The AgNPs demonstrated 98 % antibacterial inhibition at 10 mg/mL and MIC values of 0.5–1 mg/mL against Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa, comparable to gentamicin.<br><strong>Conclusions:</strong> This study presents a rapid, reproducible, and eco-friendly extracellular biosynthesis of AgNPs using a novel Bacillus strain, highlighting its potential as a sustainable nanoplatform for combating multidrug-resistant pathogens in biomedical applications. https://www.nanomedicine-rj.com/article_734215_362f4ca5e9370791f84e3d06cbac1106.pdf