Antibiotic-resistant genes traced from farms to groundwater

April 30, 2001

CHAMPAIGN, Ill. -- Genes resistant to tetracycline have been found in groundwater as far as a sixth of a mile downstream from two swine facilities that use antibiotics as growth promoters. The finding is significant in part because it shows the potential for spreading resistance back into the food chain of animals and people, researchers say.

U.S. farmers for more than 50 years have used tetracycline and other antibiotics to enhance the growth of livestock. In humans, an overuse of antibiotics is blamed for a growing resistance to many antibiotics, and agricultural use has been suspected in the spread of resistance genes. The European Union is phasing out such agricultural use; Sweden banned it in the 1980s.

Researchers from the University of Illinois and Illinois State Geological Survey used a DNA-amplification technique (polymerase chain reaction or PCR) to analyze samples from lagoons, wells and groundwater on and near two Illinois facilities, said Rustam I. Aminov, a visiting professor of animal sciences at the UI. Their research appeared in the April issue of Applied and Environmental Microbiology. Aminov had reported his creation of primers for use with PCR to detect resistance genes in the environment earlier this year in the same journal. In the earlier paper, he also reported the detection of resistance genes in livestock intestines and feces and in commercial feed.

"The use of tetracycline on farms is pushing the evolution of these genes," he said. "We found tetracycline resistance genes in soil and groundwater bacteria. The genes are transferred to this type of bacteria, where they can survive and travel long distances in the environment. It has been suggested that there is horizontal transfer of antibiotic resistance genes, but we had only seen it in laboratory experiments, not in in-situ studies. Here, we see such a transfer is occurring in the environment."

The researchers were able to identify the trail taken by the resistance genes. The DNA fingerprints in the samples matched the resistance genes previously identified in livestock and feed.

"These genes were found to be predominant in the gastrointestinal tracts of pigs and steers," the authors wrote. "The elevated frequencies of these genes in the environment surrounding the farms were consistent with the hypothesis that this occurrence was the result of gene flow from the animals."

Once resistance genes make their way into drinking water, they will find their way into the guts of the people, animals and wildlife that drink it, Aminov said. "We are potentially passing on resistance in a continuous gene cycle in the environment," he said.
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The five-member research team consisted of Aminov and Roderick I. Mackie, a professor of animal sciences; Natalie Garrigues-Jeanjean, a postdoctoral researcher in veterinary pathobiology; J.C. Chee-Sanford, now with the USDA; and Ivan J. Krapac of the State Geological Survey.

University of Illinois at Urbana-Champaign

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