The Antimicrobial Properties of Silver Nanoparticles in Bacillus subtilis Are Mediated by Released Ag+ Ions

The superior antimicrobial properties of silver nanoparticles (Ag NPs) are well-documented, but the exact mechanisms underlying Ag-NP microbial toxicity remain the subject of intense debate. Here, we show that Ag-NP concentrations as low as 10 ppm exert significant toxicity against Bacillus subtilis , a beneficial bacterium ubiquitous in the soil. Growth arrest and chromosomal DNA degradation were observed, and flow cytometric quantification of propidium iodide (PI) staining also revealed that Ag-NP concentrations of 25 ppm and above increased membrane permeability. RedoxSensor content analysis and P hag -GFP expression analysis further indicated that reductase activity and cytosolic protein expression decreased in B . subtilis cells treated with 10–50 ppm of Ag NPs. We conducted X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses to directly clarify the valence and fine structure of Ag atoms in B . subtilis cells placed in contact with Ag NPs. The results confirmed the Ag species in Ag NP-treated B . subtilis cells as Ag 2 O, indicating that Ag-NP toxicity is likely mediated by released Ag + ions from Ag NPs, which penetrate bacterial cells and are subsequently oxidized intracellularly to Ag 2 O. These findings provide conclusive evidence for the role of Ag + ions in Ag-NP microbial toxicity, and suggest that the impact of inappropriately disposed Ag NPs to soil and water ecosystems may warrant further investigation.