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Researchers Settle NanoSilver Antimicrobial Mechanism; Low Dose May Enhance Bacteria Resistance

(Beyond Pesticides, July 18, 2012) Just as the U.S. Environmental Protection Agency (EPA) opened the federal docket for the registration review of nanosilver, Rice University researchers settled a long-standing controversy over the mechanism by which silver nanoparticles, the most widely used nanomaterial in the world, kill bacteria. The researchers found that the silver ions, rather than the silver particles, have antimicrobial effects on bacteria. However, their work comes with a warning; low doses of nanosilver can make bacteria stronger and more resistant.

Silver nanoparticles are used just about everywhere, including in cosmetics, socks, food containers, detergents, sprays and a wide range of other products to stop the spread of germs. Researchers have debated the mechanisms by which nanosilver particles exert toxicity to bacteria and other organisms. They have long known that silver ions, which flow from nanoparticles when oxidized, are deadly to bacteria. In the study, “Negligible Particle-Specific Antibacterial Activity of Silver Nanoparticles,” published in NanoLetters, the researchers explain that the nanoparticles are practically benign in the presence of microbes. But when in soluble ionic form, that is, when activated via oxidation, nanosilver becomes toxic to bacteria. The research team decided to test nanoparticle toxicity in an anaerobic environment –with no exposure to oxygen — to control the silver ions’ release. They found that the filtered particles are a lot less toxic to microbes than silver ions.

Surprisingly, when the nanosilver was tested against E. coli, the team found that E. coli survival was stimulated by relatively low (sublethal) concentrations of nanosilver, suggesting a hormetic response (U-shaped dose-response curve) that would be counterproductive to antimicrobial applications. Overall, the researchers suggest that nanosilver morphological properties known to affect antimicrobial activity are indirect effectors that primarily influence silver ion release. The researchers hypothesize antibacterial activity could be controlled (and environmental impacts could be mitigated) by modulating silver ion release, possibly through manipulation of oxygen availability, particle size, shape, and/or type of coating.

NanoSilver Docket Open for Comment

EPA has published a Federal Register notice announcing the establishment of a registration review docket for nanosilver and is seeking public comment on a summary document for the registration review of nanosilver. The summary document (1) identifies nanosilver products, (2) outlines data gaps and requirements and (3) contains a preliminary workplan and fact sheet, along with ecological risk assessment problem formulation and human health scoping sections describing scientific analyses expected to be necessary to complete nanosilver review.

A silver nanoparticle consists of many silver atoms or ions clustered together to form a particle 1-300nm in size. Due to their small size, these nanoparticles are able to invade bacteria and other microorganisms and kill them. Just as the size and chemical characteristics of manufactured nanoparticles can give them unique properties, these same properties –tiny size, vastly increased surface area to volume ratio, high reactivity– can also create unique and unpredictable human health and environmental risks.

A study conducted in 2008 and confirmed by another study in 2009 shows that washing nano-silver textiles releases substantial amounts of the nanosilver into the laundry discharge water, which will ultimately reach natural waterways and potentially poison fish and other aquatic organisms. A study found nanosilver to cause malformations and to be lethal to small fish at various stages of development since they are able to cross the egg membranes and move into the fish embryos. A 2010 study by scientists at Oregon State University and in the European Union highlights the major regulatory and educational issues that they believe should be considered before nanoparticles are used in pesticides.

Nanotechnology, the science involving manipulation of materials on an atomic or molecular scale, is an emerging technology with a broad range of potential applications, such as increasing bioavailability of a drug, used in food packaging, and in cosmetics. There are hundreds of products currently on the market that contain nanomaterials of various types and functions, the most popular application being the use of nanosilver as an antibacterial substance in many consumer products. Given this, the federal government at this point is playing a game of ‘catch-up.’

A recent report finds the current level of knowledge does not allow a fair assessment of the advantages and disadvantages that will result from the use of nanopesticides, and a better understanding of the fate and effect of nanopesticides after their application is required. The suitability of current regulations should also be analyzed so that refinements can be implemented if needed. Research on nanopesticides is therefore a priority for preserving the safety of both the food chain and the environment. Similarly, the findings of a U.S. Government Accountability Office report identified concerns about the quality of environmental, health and safety research on nanotechnology.

Federal regulation of nanoparticles has been lacking. In 2011, EPA announced plans to obtain information on nanoscale materials in pesticide products. According to EPA, the agency will gather information on what nanoscale materials are present in pesticide products to determine whether the registration of a pesticide may cause unreasonable adverse effects on the environment and human health. However, thus far pending nanotechnology regulations have been placed on hold by the Office of Management and Budget (OMB).

A coalition of six consumer safety groups filed suit against the U.S. Food and Drug Administration (FDA) on December 21, 2011, citing FDA’s chronic failure to regulate nanomaterials used in these products. The lawsuit demands that FDA respond to a May 2006 petition that the coalition filed calling for regulatory actions, including nano-specific product labeling, health and safety testing, and an analysis of the environmental impacts of nanomaterials in products regulated by FDA. After years of no federal regulatory oversight, FDA in April 2012 issued two draft guidance documents addressing the use of nanotechnology by the food and cosmetics industries. The documents “encourage” safety assessments for cosmetic products containing nanomaterials, including the need for modification or development of new methods for standardized safety tests. The new guidelines for the first time show the FDA believes nanomaterials deserve greater scrutiny.

The National Organic Standards Board (NOSB) passed a recommendation directing the USDA National Organic Program (NOP) to prohibit engineered nanomaterials from certified organic products as expeditiously as possible. While there is overwhelming agreement to prohibit nanotechnology in organics generally, there is still confusion over the definition of what exactly should be prohibited and how to prohibit nanotech products in the organic industry. The recommendation deals specifically with engineered nanomaterials and purposefully omits those that are naturally occurring. Further it would block petitions seeking an exemption and keep nanomaterials out of food packaging and contact surfaces.

For more on nanosilver and nanotechnology, visit Beyond Pesticides’ program page.

Take Action: The federal register docket is now open for public comment for the registration review of nanosilver. During the comment period, anyone may submit comments, relevant data or information for the agency’s consideration. Public comment may be submitted until August 19, 2012 at www.regulations.gov in docket # EPA-HQ-OPP-2011-0370.

Source: Rice University News and Media

All unattributed positions and opinions in this piece are those of Beyond Pesticides.


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