Strains of gonorrhea have developed resistance to last-resort antibiotics, according to a recent report by the Centers for Disease Control and Prevention (CDC), making it one of a growing list of diseases that traditional antibiotics may soon be unable to cure. As the age-old war between humans and infectious microbes intensifies, one solution may be a new version of an ancient medical substance: silver.
But the silver comes in a form that doctors thousands of years ago never would have imagined—nanoparticles. A team of researchers at Nankai University in China has created a silver nanoparticle designed to be more effective against bacteria in wounds and the bloodstream while being minimally toxic to human cells. The nanoparticle has been tested in rats, and the authors claim that it could be used clinically.
While nanoparticles are a recent technology, the antimicrobial properties of silver have been known for thousands of years: the ancient Phoenicians stored liquids in silver vessels to prevent them from spoiling. “That’s why we call forks and knives silverware. You get less sick if you eat with silver,” said Georgios Sotiriou, assistant professor in the department of microbiology, tumor and cell biology at Karolinska Institutet in Sweden. More recently, silver has been used as an antimicrobial in medical applications for decades, such as in wound dressings for burn victims.
The advantage that silver nanoparticles, also called nanosilver, provide over older silver treatments is that their small size allows them to enter the bloodstream directly and release silver ions at a much higher rate. Silver ions only disperse at a particle’s surface, so the greater the ratio of surface area to volume, the more effective the particle. Nanoparticles, which have no dimension greater than 100 nanometers—roughly one thousandth the width of a human hair—are highly effective.
However, silver particles have yet to see wide use because they tend to clump together in solution, decreasing their effectiveness. To solve this problem, the team combined silver with a compound known as an alkylated ε-polylysine that they found to be especially effective at preventing clumping. Furthermore, the compound was itself a good antimicrobial agent—polylysines are used as food preservatives in Japan.
When combined, the silver and compound worked together to kill more bacteria than either could alone. The addition of the compound was “the key reason making the nanoparticles more effective,” said paper co-author Xinge Zhang, an associate professor at the Nankai University Institute of Polymer Science.
Silver nanoparticles are effective against even antibiotic-resistant strains of bacteria because they use different mechanisms than traditional antibiotics, Zhang said. The particles can kill bacteria in several ways, such as by anchoring to bacterial cell walls to tear them or by releasing silver ions that penetrate the cell membrane and deactivate important enzymes. That silver nanoparticles kill bacteria in multiple ways at once will likely make it harder for bacteria to develop resistance to them, Sotiriou said.
The new silver nanoparticle “should be effective against MRSA,” Zhang said, referring to the deadly, highly antibiotic-resistant hospital superbug.
Still, some concerns about silver nanoparticles remain.
Though bacterial resistance to silver is not common, it has been found in burn victim wards where silver is used frequently. Because of this, some scientists object to the current widespread use of nanosilver as an antibacterial ingredient in consumer products from hand sanitizers to children’s toys to deodorized socks.
Silver’s toxicity to humans is another potential concern. The Nankai research team modified the length of one part of their nanoparticle in a way that they claim minimized toxicity—but experts aren’t very concerned about toxicity in the typical medical usage of silver nanoparticles anyway.
“Silver nanoparticles applied to skin or ingested are not very toxic,” said University of Colorado Boulder assistant professor Marina Vance, who was not affiliated with the study. Vance cautioned that there are still concerns about long-term effects of silver accumulation in people and in the environment. “Introducing antimicrobial products in the wastewater stream may affect the biological process we use to treat wastewater,” Vance said.
In spite of the concerns, research groups around the world are experimenting to find new applications for silver nanoparticles. If and when the particles will be used in hospitals is hard to know. “The research is still in its early stage,” Zhang said.
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