“Zombie-effect” of Ag nanoparticles

Man has known the medicinal and preservative properties of Ag for over 6,000 years. The ancient Greek and Roman civilizations used Ag vessels to keep drinking water, to prevent microbial contaminations. Since ancient times, people have used Ag-based compounds to treat wounds, burns, and even certain bacterial diseases.

However, with the recent developments in science and technology, innovations are being made to improve the efficacy of these Ag-based bactericidal compounds. One such example is the development of Ag nanoparticles. Over the last few decades, Ag has been engineered into nanoparticles, which are minute structures from 1 to 100 nm in size. Due to their small size, the nanoparticles’ total surface area is maximized, leading to the highest values of activity to weight ratio. Because this property is distinctly different from that of the bulk metal, Ag nanoparticles have attracted much attention. They have found applications in medicine, biotechnology, textile, wastewater treatment, and even bioengineering fields.

In recent years, these Ag nanoparticles have been mainly incorporated into many consumer products like air sanitizer sprays, socks, pillows, slippers, respirators, wet wipes, detergents, soaps, shampoos, tubes of toothpaste, and air filters, owing to their pronounced antibacterial properties. One such prominent application is the incorporation of these Ag nanoparticles into surface disinfectants. Here, the Ag nanoparticles show a bactericidal effect. When these disinfectants are used on surfaces, they kill the majority of bacteria present. The mode of action or the mechanism of how Ag nanoparticles act, is of 03 types mainly.

1. Enhance the porosity of bacterial cell membranes by increasing the permeability.
2. Inhibit bacterial metabolism by blocking their respiratory pathways.
3. Inhibit bacterial cell division (reproduction) by directly binding with their genetic material.

Minimum inhibitory concentration (MIC) assays can be conducted to reveal any effect of Ag nanoparticles’ size on their bactericidal efficiency. Through these studies conducted with gram-negative E. coli and gram-positive S. aureus bacterial species, it has been discovered that the antibacterial efficacy of Ag nanoparticles tends to increase with their decreasing size. It means that the smaller the nanoparticle’s size is, the better its effect on bacteria.

On the other hand, in the case of viruses, the antiviral activity of Ag nanoparticles can be due to the direct virucidal action of the nanoparticles. For example, in the human immunodeficiency virus (HIV‐1), the Ag nanoparticles bind to the sulfur groups of gp120 protein spikes over the viral membrane, thereby preventing the viral and host cell membrane fusion. This results in the inhibition of viral infectivity, thereby rendering the viral cells harmless.

However, one of the most important discoveries regarding Ag nanoparticles’ antibacterial nature was the “zombie effect.” This effect is due to the catalytic-antibacterial nature of Ag. It means that the Argentum ions in the medium, which does not get consumed during the bacterial deactivation cycle, can kill the bacteria generation after generation until it eventually gets reduced to an inactive form. This was first demonstrated using Pseudomonas aeruginosa with silver nitrate. Here the Ag compound was used to kill a culture of this particular bacteria, and then these dead bacterial cells were used to kill a live culture of the same species. These dead bacterial cultures act as a reservoir of Argentum ions, which are transferred to the living bacteria on contact, causing their death. The bacteria accumulate small Ag nanoparticles and evenly distribute them throughout the bacterium, with Ag⁰ nanoparticles being formed by reducing Ag⁺ cations by the activity of reductive enzymes within the cells. This process continues generation after generation until all the Ag cations are reduced to Ag. Due to this, the Ag particles incorporated with disinfectants provide a long-lasting antibacterial effect.

Another factor that needs to be considered when using Ag compounds is their impact on the environment and human health. Being a heavy metal, a surplus of Ag could cause highly detrimental effects on the environment. Due to this, the release of these compounds to the environment needs to be properly managed. The majority of the adverse impacts of these Ag effluents affect marine and aquatic life.

The use of these Ag nanoparticles in consumer products to greater extents have promoted doubts in people regarding the toxicity of Ag. There is a public perception that Ag nanoparticles do not discriminate between different strains of bacteria and are likely to destroy friendly microbes that are beneficial to both humans and the environment. However, not many experiments regarding this have been done by scientists to date. It would be fair to say that the mechanism of Ag nanoparticles’ bactericidal effect is not well understood as yet. It was recently reported that “Nanosilver represents a special physicochemical system which confers their antimicrobial activities via Ag+.” If this conclusion is verified, then most bioaccumulation and toxicity issues relating to Ag nanoparticles can be considered a result of ionic silver’s toxic potential, which is a well-documented topic. This is favorable because, under natural environmental conditions, ionic silver (Ag+) is readily transformed into non-reactive compounds. This would mean that nanosilver toxicity’s environmental risks are not as severe as the popular perception may suggest. Although a fully understood hazard report of these nanoparticles would provide a major contribution to the risk assessment of consumer products that utilize them, with the data and knowledge available on the subject, It’s safe to say that the benefits of using them, especially in surface disinfectants and other antimicrobial products outweigh any adverse effects that can occur.

References:

1. Lok, C.N. et al. (2007) Silver nanoparticles: partial oxidation and antibacterial activities. J Biol Inorg Chem 12(4), 527–534
2. Ciriminna, R., Albo, Y., & Pagliaro, M. (2020). New Antivirals and Antibacterials Based on Silver Nanoparticles. ChemMedChem, 10.1002/cmdc.202000390. Advance online publication. https://doi.org/10.1002/cmdc.202000390
3. I. Linkov and J. Steevens (eds.), Nanomaterials: Risks and Benefits, 287 © Springer Science + Business Media B.V. 2009

Author :W M A N Wanasinghe
Bsc Chemistry(Hons)
University Of Colombo