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

• Published in: PloS one Vol. 10; no. 12; p. e0144306
• Main Authors: Hsueh, Yi-Huang, Lin, Kuen-Song, Ke, Wan-Ju, Hsieh, Chien-Te, Chiang, Chao-Lung, Tzou, Dong-Ying, Liu, Shih-Tung
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 15.12.2015; Public Library of Science (PLoS)
• Subjects: Absorption; Anti-Infective Agents - pharmacology; Antiinfectives and antibacterials; Antimicrobial agents; Atomic structure; Bacillus subtilis; Bacillus subtilis - drug effects; Bacillus subtilis - growth & development; Bacteria; Biodegradation; Biofilms; Chemical engineering; Chromosomes, Bacterial - metabolism; Content analysis; Crystallography, X-Ray; Data analysis; Deoxyribonucleic acid; DNA; DNA, Bacterial - metabolism; E coli; Environmental degradation; Escherichia coli; Fine structure; Flow cytometry; Green Fluorescent Proteins - metabolism; Iodides; Ions; Lymphocytes B; Materials science; Membrane permeability; Metal Nanoparticles - chemistry; Microbial Sensitivity Tests; Microorganisms; Nanoparticles; Oxidation; Oxidation-Reduction; Particle Size; Pathogenesis; Permeability; Propidium iodide; Proteins; Reductase; Reference Standards; Silver; Silver - pharmacology; Sludge; Soil investigations; Soil permeability; Soil water; Spectrum analysis; Staining and Labeling; Toxicity; Ultrastructure; X-Ray Absorption Spectroscopy; United States > US; Taiwan
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0144306

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 Phag-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 Ag2O, 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 Ag2O. 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.