title: Use of bioreporters and deletion mutants reveals ionic silver and ROS to be equally important in silver nanotoxicity.
abstract: The mechanism of antibacterial action of silver nanoparticles (AgNp) was investigated by employing a combination of microbiology and geochemical approaches to contribute to the realistic assessment of nanotoxicity. Our studies showed that suspending AgNp in media with different levels of chloride relevant to environmental conditions produced low levels of ionic silver thereby suggesting that dissolution of silver ions from nanoparticulate surface could not be the sole mechanism of toxicity. An Escherichia coli based bioreporter strain responsive to silver ions together with mutant strains of E. coli lacking specific protective systems were tested against AgNp. Deletion mutants lacking silver ion efflux systems and resistance mechanisms against oxidative stress showed an increased sensitivity to AgNp. However, the bioreporter did not respond to silver nanoparticles. Our results suggest that oxidative stress is a major toxicity mechanism and that this is at least partially associated with ionic silver, but that bulk dissolution of silver into the medium is not sufficient to account for the observed effects. Chloride ions do not appear to offer significant protection, indicating that chloride in receiving waters will not necessarily protect environmental bacteria from the toxic effects of nanoparticles in effluents.
authors.affiliation: School of Geosciences, Microbial Geochemistry Laboratory, University of Edinburgh, West Mains Road, Edinburgh EH9 3FE, United Kingdom; School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom. Electronic address: nimisha.joshi@manchester.ac.uk.
authors.forename: Nimisha
authors.initials: N
authors.lastname: Joshi
authors.affiliation: School of Geosciences, Microbial Geochemistry Laboratory, University of Edinburgh, West Mains Road, Edinburgh EH9 3FE, United Kingdom. Electronic address: Ngwenya@ed.ac.uk.
authors.forename: Bryne T
authors.initials: BT
authors.lastname: Ngwenya
authors.affiliation: School of Geosciences, Microbial Geochemistry Laboratory, University of Edinburgh, West Mains Road, Edinburgh EH9 3FE, United Kingdom.
authors.forename: Ian B
authors.initials: IB
authors.lastname: Butler
authors.affiliation: School of Biological Sciences, Institute of Cell Biology, Darwin Building, University of Edinburgh, Edinburgh EH9 3JR, United Kingdom.
authors.forename: Chris E
authors.initials: CE
authors.lastname: French
mesh_terms: D004926:Escherichia coli; D005798:Genes, Bacterial; D053758:Nanoparticles / Q000633:toxicity*; D018384:Oxidative Stress; D017382:Reactive Oxygen Species; D017384:Sequence Deletion; D012834:Silver / Q000633:toxicity*
chemical_list: D017382:Reactive Oxygen Species; D012834:Silver
keywords: Biosensor; Environment; Ionic; Mechanism; Nanomaterials; Nanotoxicity; Oxidative stress; Speciation