Superbug genes are getting into soil and water

FARMERS should stop using antibiotics as growth promoters, say researchers in the US. They have uncovered evidence of a new route by which dangerous antibiotic resistance genes can spread.

There is already strong evidence that feeding animals antibiotics can lead to the emergence of resistant strains of gut bacteria such as salmonella, which can then be passed on to people in food or through direct contact with animals. Now microbiologist Rustam Aminov of the University of Illinois at Urbana-Champaign and his colleagues have discovered that bacteria in the soil and groundwater beneath farms seem to be acquiring tetracycline resistance genes from bacteria originating in pigs' guts.

Once transferred, the resistance genes can persist in the hardier soil and water-borne bacteria and could be passed on to potentially dangerous bacteria in the environment, or in humans who drink the water. "This is very important. [The study] is the first of its kind to demonstrate this kind of broad ecological presence of tetracycline resistance genes," says Stuart Levy, director of the Center for Adaptation Genetics and Drug Resistance at Tufts University in Boston.

"And this is just tetracycline. Add all the other drugs that might be there, and then I think it further supports the notion that we should be prudent in how we use antibiotics in animals and people."

While the European Union has banned the use as growth promoters of most antibiotics that are used in human medicine, farmers in the US still routinely add antibiotics such as tetracycline, penicillin and streptomycin to livestock feed to promote animal growth. Nearly 70 per cent of all antibiotics produced in the US are fed to animals as growth promoters, according to the Union of Concerned Scientists, a non-profit organisation based in Cambridge, Massachusetts.

To study the environmental effect of these antibiotics around two swine farms that use tetracycline as a growth promoter, Aminov's team analysed samples from farm-waste lagoons and from groundwater reservoirs beneath the lagoons. They found that bacteria in the soil and groundwater carried tetracycline resistance genes, or tet genes, that were almost identical to those in bacteria living in the pigs' guts. This strongly suggests that the bacteria from the pigs are transferring their genes to the ones outside, says Aminov.

"People at both sites are drinking this groundwater without any treatment. This may be a new way of increasing the local concentration of antibiotic resistance genes and circulating them between animals, humans and the environment," he says. And as groundwater accounts for a substantial part of the public water supply in the US, the problem could be widespread.

Abigail Salyers, also at the University of Illinois, agrees. She and her colleagues recently showed that bacteria passing through human intestines exchange genes with the resident bacteria. They found that 80 per cent of the strains of a major bacterial species found in the colons of people in the late 1990s carried tetracycline resistance genes, compared with 30 per cent before 1970.

Together, the studies suggest that antibiotic resistance genes are being transferred from the environment into our bodies, she says. "What we are seeing here is that if a resistance gene gets out into the bacterial population in nature, it's like letting the genie out of the bottle. So far it looks like there are very few, if any, limits to how far a resistance gene can spread," she says.

Author: Anil Ananthaswamy
More at: Applied & Environmental Microbiology (vol 67, p 1494)

New Scientist issue 21 April 2001

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