Postbiotic-Mediated Green Synthesis of Silver Nanoparticles: Revealing Potent Antimicrobial, Anti-Biofilm, Antioxidant, and Anticancer Properties using Paenibacillus xylanexedens

This study aimed to synthesize and characterize silver nanoparticles (AgNPs) using the postbiotic of Paenibacillus xylanexedens YSM1 and to evaluate their antimicrobial, antioxidant, and anticancer activities. The extracellular postbiotic-mediated synthesis provides a simple, non-toxic, and sustainable route that eliminates the need for chemical reducing agents while enhancing nanoparticle biocompatibility. Optimization studies using UV–visible spectroscopy identified the optimal synthesis conditions as 3 mM AgNO₃ concentration, a 1:5 AgNO₃-to-postbiotic ratio, 360 min reaction time, and 60 °C incubation temperature. The distinct color change from pale yellow to dark brown confirmed AgNP formation, with a characteristic surface plasmon resonance (SPR) peak at 435 nm. FT-IR analysis revealed hydroxyl, amide, and carbonyl groups, indicating the involvement of postbiotic metabolites as natural reducing and capping agents. The biosynthesized AgNPs exhibited notable antimicrobial activity against Escherichia coli , Staphylococcus aureus , and Candida albicans , with MIC values of 62.5, 125, and 250 µg/mL, respectively. In addition, strong antibiofilm activity was observed against bacterial biofilms, achieving ≥75% inhibition for E. coli and S. aureus at concentrations≥62.5 µg/mL, while C. albicans biofilms required higher concentrations to reach comparable inhibition. The antibiofilm-effective concentrations showed close agreement with planktonic MIC values, indicating a coherent antimicrobial–antibiofilm relationship. DPPH assays demonstrated concentration-dependent radical scavenging activity, reaching nearly 100% inhibition at 500 µg/mL, confirming the antioxidant potential of postbiotic nanoparticles. Cytotoxicity studies performed on HT-29 colorectal adenocarcinoma and MRC-5 normal fibroblast cell lines revealed selective toxicity toward cancer cells (IC₅₀ ≈ 125 µg/mL) while maintaining more than 92% viability in normal cells.