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      Similarities and Differences between Silver Ions and Silver in Nanoforms as Antibacterial Agents

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          Abstract

          Silver is considered as antibacterial agent with well-known mode of action and bacterial resistance against it is well described. The development of nanotechnology provided different methods for the modification of the chemical and physical structure of silver, which may increase its antibacterial potential. The physico-chemical properties of silver nanoparticles and their interaction with living cells differs substantially from those of silver ions. Moreover, the variety of the forms and characteristics of various silver nanoparticles are also responsible for differences in their antibacterial mode of action and probably bacterial mechanism of resistance. The paper discusses in details the aforementioned aspects of silver activity.

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          Most cited references65

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          Does the antibacterial activity of silver nanoparticles depend on the shape of the nanoparticle? A study of the Gram-negative bacterium Escherichia coli.

          In this work we investigated the antibacterial properties of differently shaped silver nanoparticles against the gram-negative bacterium Escherichia coli, both in liquid systems and on agar plates. Energy-filtering transmission electron microscopy images revealed considerable changes in the cell membranes upon treatment, resulting in cell death. Truncated triangular silver nanoplates with a {111} lattice plane as the basal plane displayed the strongest biocidal action, compared with spherical and rod-shaped nanoparticles and with Ag(+) (in the form of AgNO(3)). It is proposed that nanoscale size and the presence of a {111} plane combine to promote this biocidal property. To our knowledge, this is the first comparative study on the bactericidal properties of silver nanoparticles of different shapes, and our results demonstrate that silver nanoparticles undergo a shape-dependent interaction with the gram-negative organism E. coli.
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            Negligible particle-specific antibacterial activity of silver nanoparticles.

            For nearly a decade, researchers have debated the mechanisms by which AgNPs exert toxicity to bacteria and other organisms. The most elusive question has been whether the AgNPs exert direct "particle-specific" effects beyond the known antimicrobial activity of released silver ions (Ag(+)). Here, we infer that Ag(+) is the definitive molecular toxicant. We rule out direct particle-specific biological effects by showing the lack of toxicity of AgNPs when synthesized and tested under strictly anaerobic conditions that preclude Ag(0) oxidation and Ag(+) release. Furthermore, we demonstrate that the toxicity of various AgNPs (PEG- or PVP- coated, of three different sizes each) accurately follows the dose-response pattern of E. coli exposed to Ag(+) (added as AgNO(3)). Surprisingly, E. coli survival was stimulated by relatively low (sublethal) concentration of all tested AgNPs and AgNO(3) (at 3-8 μg/L Ag(+), or 12-31% of the minimum lethal concentration (MLC)), suggesting a hormetic response that would be counterproductive to antimicrobial applications. Overall, this work suggests that AgNP morphological properties known to affect antimicrobial activity are indirect effectors that primarily influence Ag(+) release. Accordingly, antibacterial activity could be controlled (and environmental impacts could be mitigated) by modulating Ag(+) release, possibly through manipulation of oxygen availability, particle size, shape, and/or type of coating.
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              Antibacterial activity and mechanism of action of the silver ion in Staphylococcus aureus and Escherichia coli.

              The antibacterial effect and mechanism of action of a silver ion solution that was electrically generated were investigated for Staphylococcus aureus and Escherichia coli by analyzing the growth, morphology, and ultrastructure of the bacterial cells following treatment with the silver ion solution. Bacteria were exposed to the silver ion solution for various lengths of time, and the antibacterial effect of the solution was tested using the conventional plate count method and flow cytometric (FC) analysis. Reductions of more than 5 log(10) CFU/ml of both S. aureus and E. coli bacteria were confirmed after 90 min of treatment with the silver ion solution. Significant reduction of S. aureus and E. coli cells was also observed by FC analysis; however, the reduction rate determined by FC analysis was less than that determined by the conventional plate count method. These differences may be attributed to the presence of bacteria in an active but nonculturable (ABNC) state after treatment with the silver ion solution. Transmission electron microscopy showed considerable changes in the bacterial cell membranes upon silver ion treatment, which might be the cause or consequence of cell death. In conclusion, the results of the present study suggest that silver ions may cause S. aureus and E. coli bacteria to reach an ABNC state and eventually die.
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                Author and article information

                Journal
                Int J Mol Sci
                Int J Mol Sci
                ijms
                International Journal of Molecular Sciences
                MDPI
                1422-0067
                02 February 2018
                February 2018
                : 19
                : 2
                : 444
                Affiliations
                [1 ]Department of Microbiology, Institute of Genetics and Microbiology, University of Wrocław, 51-148 Wrocław, Poland; mateusz.speruda@ 123456uwr.edu.pl
                [2 ]Department of Immunology of Infectious Diseases, Hirszfeld Institute of Immunology and Experimental Therapy, Polish Academy of Sciences, 53-114 Wrocław, Poland; eva.krzyzewska@ 123456iitd.pan.wroc.pl (E.K.); rybka@ 123456iitd.pan.wroc.pl (J.R.)
                [3 ]Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-422 Wrocław, Poland; a.lukowiak@ 123456int.pan.wroc.pl
                Author notes
                [* ]Correspondence: anna.kedziora@ 123456uwr.edu.pl (A.K.); gabriela.bugla-ploskonska@ 123456uwr.edu.pl (G.B.-P.); Tel.: +48-71-375-62-22 (A.K.); +48-71-375-63-23 (G.B.-P.)
                Author information
                https://orcid.org/0000-0002-5581-1690
                Article
                ijms-19-00444
                10.3390/ijms19020444
                5855666
                29393866
                75308c39-b8c0-423e-b1fa-e7e9c54765c3
                © 2018 by the authors.

                Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/).

                History
                : 02 December 2017
                : 16 January 2018
                Categories
                Review

                Molecular biology
                silver ions,silver nanoparticles,silver nanocompounds,nanotechnology,mode of action,resistance of bacteria

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