Copper Could Inspire Microbes To Resist

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Copper conundrum Use of copper in pesticides and in animal feed could help soil-dwelling microbes resist antibiotics.

For decades, microbes in soils have shown signs of resistance to antibiotics used in medicine and agriculture, raising concerns of possible human health impacts. New research in Environmental Science & Technology DOI: 10.1021/es101798r) indicates that copper, which is widely used in pesticides and animal feed, might prime microbes for such resistance.

In the past two decades, researchers have shown that microbes can develop resistance to metals in soils as a protective reaction against metal toxicity. Kristian Brandt of the University of Copenhagen and his colleagues studied soil contaminated nearly a century ago by a processing plant that treated wood to make it last longer using copper sulfate. While records are spotty, the team determined that the contaminated soils were nearly identical to nearby control soils:  Both underwent the same tilling practices in the decades after the wood plant closed. While both soils had similarly low levels of other metals and pollutants, copper levels were 150 times higher in the processing-plant contaminated soils.

The team used a novel radiolabeled assay to detect which microbial populations survived in the lab in the presence of copper and antibiotics.  Compared to bacteria from the normal soil, the microbes that came from the copper-contaminated soils showed higher resistance to vancomycin and tetracycline, antibiotics commonly used in agriculture.

Brandt and his colleagues previously showed that microbes can develop resistance to copper, even at low levels, in about a week. Resistance to antibiotics also could develop quickly, the researchers hypothesize. More worrisome, they suggest that copper exposure may make microbes resistant to as-yet undeveloped drugs.

While Gerry Wright of McMaster University, in Ontario, is intrigued by the link between copper and antibiotic resistance, he says that the researchers have more work to do: They have to find the mechanism. "What they've got is a phenomenon that needs to be pursued," he says.

For instance, the researchers haven't shown whether copper triggers genetic changes that can be passed on. David Graham, an environmental engineer at Newcastle University, in England, who also studies historically contaminated sites, suggests that the changes may be temporary. As soon as the microbes are taken away from their "home" soils, and away from the influence of copper, he says, they may quickly revert to being less-super bugs. 

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