The surprising cause of antibiotic resistance

Near the town of Aiken, in southwestern South Carolina, is a remnant of the pre-antibiotic era. It’s known as the Savannah River Site, a 310-square-mile area where the U.S Department of Energy once manufactured materials for nuclear weapons.

“The site was constructed and closed to the public before antibiotics were used in medical practices and agriculture,” says J. Vaun McArthur, an ecologist at the University of Georgia’s Savannah River Ecology Laboratory and Odum School of Ecology. Yet the site’s streams harbor high levels of antibiotic resistant bacteria, McArthur and his colleagues reported recently in the journal Environmental Microbiology.

Overuse or misuse of antibiotics is usually thought of as the cause of antibiotic resistance. But the new study suggests something else is afoot. McArthur suspects toxic industrial contaminants, especially heavy metals like cadmium and lead, can also contribute to the spread of antibiotic resistance.

McArthur and his team collected 427 samples of Escherichia coli, a bacterium commonly found in the guts of humans, other mammals, and birds, from 11 locations on nine streams within the Savannah River Site. They tested these samples against five common antibiotics.

Bacteria from the sediments of three streams showed the highest levels of antibiotic resistance. One of these streams, Upper Three Runs Creek, was sampled near the spot where the stream system enters the site. So its waters likely have exposure to antibiotics from residential and agricultural areas upstream.

But the other two streams, U4 and U8, are located entirely within the site. “The streams have not had inputs from wastewater, so we know the observed patterns are from something other than antibiotics,” McArthur says. However, these streams are located in areas of the site that are contaminated with industrial waste.

The researchers also tested bacteria from U4, U8, and another stream, U10, against a panel of 23 combinations of antibiotics. Again they found widespread resistance, even to recently introduced classes of antimicrobials. But the contaminated streams, U4 and U8, had much greater antibiotic resistance than U10, which is relatively untouched by industrial contamination.

Meanwhile, samples from 6 pristine streams were susceptible to a broad range of antibiotics, McArthur reports. The findings suggest that industrial pollution is not only harmful in its own right, but may also pose a much broader threat to our ability to treat infectious disease.

Other researchers have also found antibiotic resistance without an obvious source of antimicrobials, and higher antibiotic resistance associated with industrial areas. But the new study provides a particularly convincing demonstration of the potential links. “The findings of this study may well explain why resistant bacteria are so widely distributed,” McArthur says.  – Sarah DeWeerdt | January 12, 2016

Source: McArthur J.V. et al. “Patterns of multi-antibiotic-resistant Escherichia coli from streams with no history of antimicrobial inputs.” Environmental Microbiology DOI: 10.1007/s00248-015-0678-4

Header image: Escherichia coli, a common intestinal bacterium. Patterns of antibiotic resistance among samples of this bacterium from South Carolina suggest that industrial pollution may threaten the effectiveness of antibiotics. Credit: James Joel via Flickr.

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