title: The Antimicrobial Properties of Silver Nanoparticles in Bacillus subtilis Are Mediated by Released Ag+ Ions.
abstract: 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.
authors.affiliation: Graduate School of Biotechnology and Bioengineering, Yuan Ze University, Taoyuan, Taiwan.
authors.forename: Yi-Huang
authors.initials: YH
authors.lastname: Hsueh
authors.affiliation: Department of Chemical Engineering and Materials Science, Yuan Ze University, Taoyuan, Taiwan.
authors.forename: Kuen-Song
authors.initials: KS
authors.lastname: Lin
authors.affiliation: Graduate Institute of Biomedical Sciences, and Research Center for Bacterial Pathogenesis, Chang Gung University, Taoyuan, Taiwan.
authors.forename: Wan-Ju
authors.initials: WJ
authors.lastname: Ke
authors.affiliation: Department of Chemical Engineering and Materials Science, Yuan Ze University, Taoyuan, Taiwan.
authors.forename: Chien-Te
authors.initials: CT
authors.lastname: Hsieh
authors.affiliation: Department of Chemical Engineering and Materials Science, Yuan Ze University, Taoyuan, Taiwan.
authors.forename: Chao-Lung
authors.initials: CL
authors.lastname: Chiang
authors.affiliation: Department of Chemical Engineering and Materials Science, Yuan Ze University, Taoyuan, Taiwan.
authors.forename: Dong-Ying
authors.initials: DY
authors.lastname: Tzou
authors.affiliation: Graduate Institute of Biomedical Sciences, and Research Center for Bacterial Pathogenesis, Chang Gung University, Taoyuan, Taiwan.
authors.forename: Shih-Tung
authors.initials: ST
authors.lastname: Liu
mesh_terms: D000890:Anti-Infective Agents / Q000494:pharmacology*; D001412:Bacillus subtilis / Q000187:drug effects* / Q000254:growth & development; D002876:Chromosomes, Bacterial / Q000378:metabolism; D018360:Crystallography, X-Ray; D004269:DNA, Bacterial / Q000378:metabolism; D049452:Green Fluorescent Proteins / Q000378:metabolism; D007477:Ions; D053768:Metal Nanoparticles / Q000737:chemistry*; D008826:Microbial Sensitivity Tests; D010084:Oxidation-Reduction; D010316:Particle Size; D012015:Reference Standards; D012834:Silver / Q000494:pharmacology*; D013194:Staining and Labeling; D056928:X-Ray Absorption Spectroscopy
chemical_list: D000890:Anti-Infective Agents; D004269:DNA, Bacterial; D007477:Ions; D049452:Green Fluorescent Proteins; D012834:Silver