Antibiotic-eating bug unearthed in soil

IT’S WELL KNOWN how bacteria exposed to antibiotics for long periods will find ways to resist the drugs—by quickly pumping them out of their cells, for instance, or modifying the compounds so they’re no longer toxic.

Now new research has uncovered another possible mechanism of antibiotic “resistance” in soil. In a paper published on Dec. 6 in the Journal of Environmental Quality, a group of Canadian and French scientists report on a soil bacterium that breaks down the common veterinary antibiotic, sulfamethazine, and uses it for growth.

Certain soil bacteria are already known to live off, or “eat,” agricultural pesticides and herbicides, says the study’s leader, Ed Topp, a soil microbiologist with Agriculture and Agri-Food Canada in London, Ontario. In fact, the microbes’ presence in farm fields can cause these agrichemicals to fail.

But to Topp’s knowledge, this is the first report of a soil microorganism that degrades an antibiotic both to protect itself and get nutrition.

“I think it’s kind of a game changer in terms of how we think about our environment and antibiotic resistance,” he says.

Concerns about widespread antibiotic resistance are what led Topp and his collaborators to set up an experiment 14 years ago, in which they dosed soils annually with environmentally relevant concentrations of three veterinary antibiotics: sulfamethazine, tylosin, and chlortetracycline. Commonly fed to pigs and other livestock, antibiotics are thought to keep animals healthier. But they’re also excreted in manure, which is then spread once a year as fertilizer in countless North American farm fields.

The researchers first wanted to know whether these yearly applications were promoting higher levels of antibiotic resistance in soil bacteria. But a few years ago, they also decided to compare the persistence of the drugs in soil plots that had been repeatedly dosed, versus fresh soils where antibiotics were never applied.

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