Antimicrobial Activity of Silver Ions and Nanoparticles – A Comparative Study

Objectives: The antimicrobial properties of silver-containing compounds, including silver ions (Ag⁺) and silver nanoparticles (AgNPs), are well established. However, it is worth noting their mechanisms of action are different, with Ag⁺ directly interacting with thiol groups in enzymes and proteins, while AgNPs damage cell membranes and possibly form free radicals. Meanwhile, few studies have been aimed at comparing the antimicrobial activity of Ag⁺ and AgNPs using the same protocols. Our aim was to compare the antimicrobial activity of Ag⁺ produced by electrolysis to that of AgNPs against S. aureus, a common wound contaminant.
Methods: Ag⁺ solutions with different concentrations ( Ag⁺ 6, 7, 8ppm) were produced by electrolysis of silver metal. Different concentrations of AgNPs solutions (AgNPs 160, 180, 200ppm) were prepared by a chemical reduction method. Nanoparticle morphology and concentration were investigated by transmission electron microscopy (TEM) and ion selective electrode (ISE) after digestion with H₂O₂. The minimum inhibitory concentration (MIC) was measured by a broth microdilution method. Medium broth served as a negative control. Bacterial growth curves up to 36h were obtained from the optical density (OD) at 600 nm. Experiments were conducted in triplicate.
Results: TEM revealed spherical AgNPs with a mean diameter of 12.5±2.7nm. The MIC at 36h was 8±0.00ppm for Ag⁺ compared to 200±14ppm for AgNPs (p<0.05). The bacterial growth (lag phase) was extended from 6h to 22h with Ag⁺ 6ppm, while AgNPs 180ppm was necessary to produce the same inhibitory effect. Statistically significant reductions (p<0.0001) in OD₆₀₀ at 36h were observed for Ag⁺ 8ppm and AgNPs 200ppm (0.00±0.00), followed by Ag⁺ 7ppm (0.06±0.01), AgNPs 180ppm (0.20±0.01), Ag⁺ 6ppm (0.26±0.01), and AgNPs 160ppm (0.31±0.01), respectively.
Conclusions: Ag⁺ generated from the electrolytic method exhibited significantly higher antimicrobial activity than AgNPs, with a MIC 25 times lower than that of AgNPs.