Antibacterial Activity of Silver Nanoparticles: Structural Effects

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.

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.

Nanotechnology is a most promising field for generating new applications in medicine. However, only few nanoproducts are currently in use for medical purposes. A most prominent nanoproduct is nanosilver. Nanosilver particles are generally smaller than 100nm and contain 20-15,000 silver atoms. At nanoscale, silver exhibits remarkably unusual physical, chemical and biological properties. Due to its strong antibacterial activity, nanosilver coatings are used on various textiles but as well as coatings on certain implants. Further, nanosilver is used for treatment of wounds and burns or as a contraceptive and marketed as a water disinfectant and room spray. Thus, use of nanosilver is becoming more and more widespread in medicine and related applications and due to increasing exposure toxicological and environmental issues need to be raised. In sharp contrast to the attention paid to new applications of nanosilver, few studies provide only scant insights into the interaction of nanosilver particle with the human body after entering via different portals. Biodistribution, organ accumulation, degradation, possible adverse effects and toxicity are only slowly recognized and this review is focusing on major questions associated with the increased medical use of nanosilver and related nanomaterials.