Negligible Particle-Specific\nAntibacterial Activity\nof Silver Nanoparticles

Abstract

For nearly a decade, researchers have debated the mechanisms\nby\nwhich AgNPs exert toxicity to bacteria and other organisms. The most\nelusive question has been whether the AgNPs exert direct “particle-specific”\neffects beyond the known antimicrobial activity of released silver\nions (Ag⁺). Here, we infer that Ag⁺ is the definitive\nmolecular toxicant. We rule out direct particle-specific biological\neffects by showing the lack of toxicity of AgNPs when synthesized\nand tested under strictly anaerobic conditions that preclude Ag(0)\noxidation and Ag⁺ release. Furthermore, we demonstrate\nthat the toxicity of various AgNPs (PEG- or PVP- coated, of three\ndifferent sizes each) accurately follows the dose–response\npattern of <i>E. coli</i> exposed to Ag⁺ (added\nas AgNO₃). Surprisingly, <i>E. coli</i> survival\nwas stimulated by relatively low (sublethal) concentration of all\ntested AgNPs and AgNO₃ (at 3–8 μg/L Ag⁺, or 12–31% of the minimum lethal concentration (MLC)),\nsuggesting a hormetic response that would be counterproductive to\nantimicrobial applications. Overall, this work suggests that AgNP\nmorphological properties known to affect antimicrobial activity are\nindirect effectors that primarily influence Ag⁺ release.\nAccordingly, antibacterial activity could be controlled (and environmental\nimpacts could be mitigated) by modulating Ag⁺ release,\npossibly through manipulation of oxygen availability, particle size,\nshape, and/or type of coating.