Biogenic Silver Nanoparticles Synthesised Using Staphylococcus saprophyticus Cell-Free Supernatant Exhibit Potent Antimicrobial Activities via Oxidative Stress.

In this study, we report the eco-friendly synthesis of silver nanoparticles (AgNPs) using cell-free supernatant of Staphylococcus saprophyticus as a biological reducing and stabilising agent. Nanoparticle formation was confirmed by visible colour change and UV-visible spectroscopy (λmax at 418 nm). Structural characterisation by SEM, XRD and FT-IR indicated predominantly spherical, crystalline, silver-containing nanoparticles associated with extracellular biomolecules that may contribute to stabilisation; XRD analysis indicated a surface phase dominated by silver halides (AgBr/AgCl) rather than confirming a purely metallic Ag0 lattice. The biosynthesised AgNPs exhibited strong antimicrobial activity against Staphylococcus aureus and Pseudomonas aeruginosa, with minimum inhibitory concentrations of 3 and 0.5 μg/mL, respectively. Significant antifungal activity was also observed against Candida albicans, with marked growth suppression at 100 μg/mL. The AgNPs effectively reduced biofilm formation, extracellular matrix production and virulence traits. DCFH-DA fluorescence and MTT assays showed increased reactive oxygen species generation and reduced cellular metabolic activity in treated microbes, suggesting oxidative stress-associated antimicrobial effects. Cytotoxicity testing indicated low toxicity towards HeLa cells at lower concentrations, with dose-dependent effects observed only at higher exposure levels. Overall, results demonstrate that biogenic AgNPs possess antibacterial, antifungal and antibiofilm properties, highlighting their potential as sustainable and eco-friendly antimicrobial candidates.