Biological Interactions of Silver
The first tangible evidence of the use of silver in water purification comes from the discovey of a silver salver from the Chaldean Empire (ca. 2850 BC), which is now exhibited in the Louvre in Paris. Early civilisations discovered that water preserved in silver vessels remained much purer and more acceptable for drinking than in earthenware pitchers. The disinfecting powers of silver nanoparticles may extend back to Babylonian times when silver containers were used to transport water for the personal use of the Kings of Persia. Silver is still extensively used in water purification today, for example hot water systems in hospitals use copper-silver filters as a protection against Legionella species and MRSA infections.
Silver has a long standing and increasing value in medicinal applications from at least the middle ages. In 1893 Carl von Nägeli, a Swiss botanist, is credited with the discovery that silver inhibits the growth of bacteria at exceedingly low concentrations. This concept has become known as “oligodynamic action”. A vast amount of research has been conducted since then, and it has been found that silver has the capacity to absorb (or adsorb) to metallic and non-metallic substances or surfaces for use in medical devices, without losing its antibiotic properties.
Significant advances have been made in the use of silver and its compounds in medicial science and surgery. In the periodic table of the elements, silver has been shown to have three oxidation states, Ag+, Ag++ and Ag+++, although only the Ag+ state has been shown to be of relevence as an antimicrobial agent in medical devices and textilles.
Silver is not inert in the presence of biological materials as thought at one time, but it has been established that, whereas metallic silver is largely inert, it slowly ioinses in the presence of moisture, body fluids and secretions to release biologically active Ag+. This ion readily binds to proteins and cell surface receptors on cell membranes .
Silver nanoparticles (AgNP) have found their way as a microbiocide for biological surfaces in various forms such as wound dressings, medical devces, deodorant sprays and fabrics. Several studies have shown the potent antiviral action of AgNP’s against human pathogenic viruses such as Respiratory Syncitial Virus (RSV), Influenza Virus, Norovirus, Hepatitus B Virus (HPV) and Human Imminodeficiency Virus (HIV). Ag has also been demonstated to inactivate SARS-CoV.
Potent Antiviral Effect on SARS-CoV-2
In 2020, research was carried out using colloidal silver (cAg) and elemental Ag nanoparticles of different diameters . Samples of SARS-CoV-2 were obtained from NIID (National Institute of Infectious Diseases), Japan, stored in containers at -80 oC and handled in biosafety level 3. A plaque assay was performed on various cell lines, using crystal violet to visualise the plaques against a white background, and infected dose (TCID50) and multiplicity of infection (MOI) were calculated . Plaques are readily visible where the cells have been destroyed by viral infection. A cell viability assay using a luminescence technique was used which detects live cells based on adenosine triphosphate (ATP) levels. Samples of Ag were introduced, and cell death measured using luminescence intensities. Various assays were used to determine silver and SARS-CoV-2 interaction.
AgNP’s exhibit cytotoxicity to mammalian cells by generating reactive oxygen species. To determine the concentration required to exhibit cytotoxity, cAg was diluted and added to the cells (both human and non-human origin). Cell viability was assessed after 48 hours using a Cell-Glo assay. Ag was found to exhibit cytotoxicity at concentrations of 20 ppm and above for both types of cell lines. To ascertain the effect of AgNP’s on SARS-CoV-2, the multiplicity of infection (MOI) was calculated by independent experiments and was found to be 0.5 for human cells, and 0.05 for non-human cells.
Viral suspension at a fixed MOI was treated with each dilution of cAg for one hour and then inoculated to the cell lines. Cell viability was assessed after 48 hours to identify the proportion of cells killed by the virus and the viral load calculated. After 96 hours cAg showed robust antiviral effect denoted by increased viability of infected cells, in concentrations ranging from 1 to 10 ppm.
Since 2 ppm was 10-fold lower than the cytotoxic concentraion, it was chosen as the desired concentration for further studies. In previous studies, it has been shown that there is a size dependency of AgNP’s for viral inactivation, and that particle size less than or equal to 10 nm being reported to have maximum antiviral effect. The virus was therefore treated with 2 ppm solution of AgNP’s of different sizes for one hour and the virus-AgNP mixture was added to the cell lines. Antiviral effect was noted with AgNP’s rangng from 2 to15 nm (nanometers).
Considering the mechanism of action of AgNP’s shown by other authors, it can be presumed that AgNP’s exert their antiviral effect on SARS-CoV-2 by disrupting disulphide bonds on the spike protein and ACE2 receptors. Several studies have shown the size dependent antiviral effect of AgNP’s with particles around 10 nm in diameter being most effective. Consistent with this, the authors observed that anti-SARS-CoV-2 activity only with AgNp’s of diameters ranging from 2 to 15 nm.
Study of Orally Dosed Colloidal Silver
A study by Munger et al in 2014 used 10 ppm oral silver particle dosing with 3-, 7-, and 14-day time periods. After the completion of each time period, an independent Data Safety and Monitoring Board (DSMB) reviewed every measurement for evidence of toxicity .
No clinically important changes in metabolic, hemotalogic, urinalysis measures were identified. Also, no morphological changes were detected in the lungs, heart or abdominal organs. No significant changes were noted in pulmonary reactive oxygen species or pro-inflammatory cytokine generation.
The study demonstrates that 14-day monitored human oral dosing of a commercial oral nanoparticle silver colloidal product does not produce observable clinically important toxic effects.
From the two studies above, the specification for a colloidal silver preparation (cAg) would be a concentration of 10 ppm, and particle sizes between 2 and 15 nm.
This is consistent with colloidal silver made using the PyraMed colloidal silver generator, since the ppm is controlled by firmware on the microcontroller chip so that an automatic shutoff operates when the concentration has reached 12 ppm. After filtering and bottling, the resulting solution is 10 ppm. An electron micrograph of this solution also shows a suitable range of particle sizes:
- Lansdown, Alan B.G., 2010, Silver in Healthcare, Its Antimicrobial Efficacy and Safety in Use, RSC Publishing.
- Sundararaj, Jeremiah S., et al, 2020. Potent Antiviral Effect of Silver Nanoparticles on SARS-CoV-2, Biochemical and Biophysical Research Communications. 553(1), pp. 95-200.
- Detecting Viruses, The Plaque Assay. Virology Blog
- Munger M et al. 2014. In vivo human time-exposure study of orally dosed commercial silver nanoparticles, Nanomedicine: Nanotechnology, Biology, and Medicine. 10(1), pp.1-9.