Postbiotic-Derived Silver Nanoparticles from Enterococcus faecium: A Promising Green Alternative for Nanoparticle Synthesis with Biopreservative Effects and Antimicrobial Activity

Postbiotics are defined as microbial metabolic products. Consequently, offer a promising green alternative for nanoparticle synthesis. Accordingly, three postbiotic fractions of Enterococcus faecium were used to facilitate the green biosynthesis of silver nanoparticles (AgNPs) comprising the filter supernatant (FS), ultrasonicated pellet supernatant (UPS), and their mixture (FUS). This entailed the stepwise optimization of extract-to-metal ratio, pH, temperature, and incubation time. Two drying methods were then tested, lyophilization and oven drying, along with their effects on biological properties. Lyophilized nanoparticles (LNs) significantly enhanced antimicrobial and antioxidant activities compared with oven-dried counterparts. LNs were further characterized using UV–Vis spectroscopy, SEM–EDX, TEM–EDX, DLS, zeta potential, FTIR, and XRD analyses. All samples consisted of crystalline AgNPs coated with biomolecule-rich coronas. Among them, lyophilized FUS-derived nanoparticles (LFUS-AgNPs) showed superior morphological features, uniform size distribution, high colloidal stability, good hemocompatibility, strong radical-scavenging activity, and broad-spectrum antibacterial efficacy against eight foodborne pathogens. Bacteriostatic effects of LFUS-AgNPs against Escherichia coli and Bacillus cereus exerted concentration-dependent growth kinetic retardation. The antimicrobial activity of LFUS-AgNPs was determined in the in vitro test in commercial skim UHT milk as a food model. The two foodborne pathogens were used to assess the effectiveness of LFUS-AgNPs at 25°C and 4°C over a period of 15 days. At 4°C, LFUS-AgNPs achieved more than 5-log reductions within 7 days, while more modest but consistent inhibitory effects were exerted at 25°C. Present findings suggest that postbiotics can be a promising green alternative for nanoparticles with antimicrobial activity and a food preservative.