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Widening Use of Silver Microbicides Raises Several Sets of Concerns

Silver is an age-old, effective microbicide, but one whose commercial use is growing way too rapidly, says Samuel Luoma of the University of California, Davis. Consumer products, including socks, underwear, towels, toothbrushes, paper towels, teddy bears, combs for pets, and food containers, are among the many objects now being embedded with silver nanoparticles to render them microbially resistant. The antimicrobial potency of silver makes it particularly valuable in hospitals and other health care settings, adds Simon Silver of the University of Illinois in Chicago (UIC). However, they and other experts now point to concerns over silver nanoparticles leaching from such products into the environment plus a collateral concern that a backlash to its high use in consumer products could lead to severely restricted uses in health care settings. Silver is now being fashioned into nanoparticles, a change that accentuates its antimicrobial properties, according to Luoma, who recently completed amonograph on this subject for the Woodrow Wilson International Center for Scholars. "It is also among the most toxic of the metals to plants like phytoplankton, as well as to invertebrates and fish," he says. "Picture a silver nanoparticle at the cell membrane. It can release silver ions immediately to transporters into the cell. One reason they are such strong toxins to prokaryotes is that a lot of [their vulnerable] machinery is in the cell membrane."

For all its toxicity to microbes, how much a threat silver nanoparticles pose to public health and the environment is not known, according to Luoma. Before the photographic industry became environmentally conscious during the 1970s, use of black-and-white film development procedures routinely released large quantities of silver particles into wastewater and then the open environment, Luoma points out. Thus, he and others began to study silver and its effects in the San Francisco Bay. Although these researchers tracked the collapse of the bivalve population, which formed much of the base of the fish and bird food webs, "we did not study the microbial community," he says.

Some observers worry that silver nanoparticles will poison wastewater treatment plants. Doing so "would take massive uses of silver in the commercial sector," Luoma says. However, if silver ion dishwashers, washing machines, and air conditioners come into wide use, leached levels of nanoparticles might bring this nowremote possibility closer to reality.

In terms of direct risks to humans, "all indications are you would have to have quite a lot of silver in your system to have any impact," he adds. Further, some people drink colloidal silver as a dietary supplement, and "seem to be healthy." Silver toxicity comes within its own early warning system. Thus, a blue tinge, called argeria, appears on skin before other ill effects are manifest. What about microorganisms developing resistance to silver? Products containing silver are used in dressings for trauma wounds, burns, and diabetic ulcers, according to Silver from UIC. While resistance genes are found in bacteria associated with such wounds, including methicillin-resistant
Staphylococcus aureus
, the silverimpregnated dressings remain effective, according to Christine Cochrane at Liverpool John Moores University in Liverpool, United Kingdom, and her collaborators.

In 2008, activists led by the Washington, D.C.-based Center for Food Safety and the International Center for Technology Assessment petitioned the U.S. Environmental Protection Agency (EPA), urging it to regulate silver nanoparticles as a pesticide. In November 2009, EPA convened one of its advisory panels, asking these outside experts to assess levels of exposure and hazard from current uses of silver nanoparticles, and to advise the agency on how or whether to regulate such products and protect public health and the environment. The agency is not yet saying what comes next. "There are currently no plans for rulemaking," says Dale Kemery, an EPA press officer, referring to silver nanoparticles. "We cannot comment on any aspect of the [scientific advisory panel] until their report is released."

"It's hard to work out how serious this is," Luoma says. "The biggest concern is the potential for encouraging resistance to develop in certain strains of microbes, and the possibility of impacting the ecosystem at a fairly low level if a lot of this is released." If nothing else, concerns over rampant use of silver nanoparticles echoes those arising from profligate antibiotic use. At the very least, improved public education is warranted in both cases.

David Holzman
David Holzman is the Microbe Journal Highlights Editor.

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