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A Silver Coating in the Fight Against Microbes

Silver nanoparticles could be the next step forward in antibacterial products.



Luis Carlos Torres/iStockphoto

A new technique in paint making could soon make almost any surface germfree. Researchers have made paint that is embedded with silver nanoparticles known for their ability to kill bacteria and other microbes, in the hope that hospitals will coat their walls and countertops to fight infection.

According to the U.S. Centers for Disease Control and Prevention (CDC), more than one million people a year contract bacterial infections in hospitals. Silver itself is an excellent bacteria fighter, and in nanoparticle form it is even more potent at killing microorganisms. So far it has not shown any adverse effects in humans.

Some scientists, however, are concerned that silver nanoparticles may not be as harmless as they appear. Little research has been done on their health and environmental effects, and silver kills good microorganisms along with the bad. Also, there are currently no restrictions on using silver nanoparticles, which are already popping up in a range of consumer products that tout their antibacterial properties.

"Nanoparticles are very small and they are interacting with the bacteria and rupturing the cell wall," says chemist George John of The City College of New York and lead author of the study, published recently in the journal Nature Materials. This rupturing kills the bacteria, he explains.

A silver nanoparticle is a small cluster of silver atoms less than 100 nanometers, or 100 billionths of a meter, wide. Because of their size, nanoparticles exhibit different properties than their bulkier counterparts. They react more readily with their surroundings, which makes them dissolvable in paint. Nanoparticles are also being studied for their potential medical uses, particularly in drug delivery, because they are able to pass easily through cell membranes.

Silver has long been known to be a good antimicrobial, and nanoparticles consisting of this metal are no different. John tested the paint on both Escherichia coli and Staphylococcus aureus bacteria. In both cases, when the strains were added to a glass slide coated with the silver-infused paint and incubated at favorable conditions, there was no growth of either organism. In contrast, slides without the paint and slides with silver-free paint both showed bacterial growth.

"It is more or less like a soaping or detergent effect," says Lucian Lucia, associate professor of chemistry at North Carolina State University. The nanoparticle destroys the cell wall of the microbe.

Lucia and John both agree that bacteria cannot build up a resistance to silver nanoparticles as they can to antibiotics, because of the way the it attacks—destroying the physical structure of the cells, which kills them. Antibiotics, on the other hand, suppress the activity of bacteria but don't necessarily exterminate them. "That's the beauty of silver," Lucia says. "There's no way to develop a resistance to it."

John says he is also experimenting with different size nanoparticles. Changing the size also changes the color. So, a blue paint would use different size nanoparticles than a red paint. Currently, the size of the silver nanoparticles he is using turns the paint yellow.

The next step is to do more health and safety tests and to determine how long the paint retains its bactericidal properties. John believes it will keep its germ-killing abilities for up to three years but says it could be longer.

Silver's ability to kill bacteria has long been known, but not everyone is sold on the idea of using its nanoparticle version in consumer products. Limited research has been done on how long the nanoparticles keep their antimicrobial properties and how they interact with other organisms, which is critical because of the particles' ability to penetrate cell membranes. Some people may be uncomfortable lathering on sunscreen if it contains silver nanoparticles.

"Certainly it is a very good antimicrobial product," says Zhiqiang Hu, a civil and environmental engineer at the University of Missouri–Columbia, who is studying the safety of silver nanoparticles. "But, it can kill the benign species [of bacteria] as well."

Hu says one of his major concerns is their potential effect on aquatic organisms. Many types of bacteria live in lakes and streams, and if silver nanoparticles were to get into these waters they could disrupt the aquatic ecosystem.

Hu is not the only one worried. Andrew Maynard, chief science advisor for the Project on Emerging Nanotechnologies, funded by the Woodrow Wilson International Center for Scholars and The Pew Charitable Trusts, is also concerned about the lack of research and regulation on the use of silver nanoparticles. He says this technology is cropping up in unlikely products, such as socks, kitchenware and cosmetics, to name a few.

"You have an antimicrobial agent appearing everywhere, including children's fluffy toys, with no knowledge about its health or environmental implications," Maynard says. "What are the chances of it taking out an ecologically important bacteria?"

It is this question that Maynard wants answered before the technology is applied to any more commercial products. On the other hand, Maynard acknowledges that the use of silver nanoparticles holds promise, particularly in hospital settings.

"I think there are multiple places in which it would be okay," Maynard says. Treating patients with wounds or creating a sterile environment in a hospital are two examples of what he sees as a good use.

"Silver is one of our best lines of defense against a number of microbes," he says. "And we need to think carefully before we put such a powerful agent in the market."

This article is provided by Scienceline, a project of New York University's Science, Health and Environmental Reporting Program.

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