Weissella Cibaria Exopolysaccharides Confer Gram‐Selectivity to Photoreductively Synthesized Silver Nanoparticles

Silver nanoparticles (AgNPs) were synthesized by UVA‐photoreduction using the photoinitiator Irgacure‐2959 and exopolysaccharides (EPS) from Weissella (W.) cibaria FMy 2–21–1 as stabilizers. The AgNPs exhibited a surface plasmon resonance at 400 nm, spherical morphology (13 ± 2 nm by TEM), and a zeta potential of ‐23 ± 0.6 mV. FTIR analysis indicated that EPS functional groups (O─H, C─O─C) contributed to AgNPs stabilization. Antimicrobial activity of the EPS‐stabilized AgNPs (AgNP@EPS) was strain‐dependent: AgNP@EPS (2.83 nM; synthesized with 2 mg/mL EPS) showed potent activity exclusively against Gram‐positive bacteria, with the greatest efficacy against Micrococcus luteus (inhibition zone: 13 ± 1 mm; MIC 0.10 nM). Mechanistic studies suggest the antibacterial effect is mediated by generation of reactive oxygen species (ROS), with no significant Ag⁺ ion release or overt membrane disruption. The observed selectivity for Gram‐positive bacteria is attributed to strong electrostatic interactions, reflected by a marked change in bacterial zeta potential after treatment. To our knowledge, this is the first report of AgNPs produced by photoreduction using EPS from W. cibaria ; this approach affords improved control over nanoparticle characteristics compared with conventional green synthesis methods. These bio‐stabilized AgNPs show promise as selective antimicrobial agents.