When antibiotics first became available, farmers used them indiscriminately—dribbling streptomycin into chicken feed to boost growth and doling out low doses to fatten pigs. Now scientists know that the overuse of antibiotics in livestock can foster drug-resistant bacteria that are dangerous to human health. Amid debates over what kinds of restrictions should be put in place, figuring out how antibiotic-resistant bacteria evolve and make their way to humans remains an area of intense interest.

Jo Handelsman is tracing one such pathway that, as she puts it, travels from “barn to table.” Handelsman, a microbiologist who is now associate director for science at the White House Office of Science and Technology Policy, looked into dairy cows, which are often treated with antibiotics and produce manure that farmers use on their crops. In addition to nutrients, that fragrant fertilizer may harbor antibiotic-resistant bacteria—a problem because the microbes can come into contact with plants that are subsequently shipped to supermarkets and sometimes eaten raw.

To tease out how those antibiotic-resistant bacteria come to exist, Handelsman and her colleagues at Yale University added manure from a nearby Connecticut farm to raised beds of soil in 2013. In this case, the manure specifically came from cows that were not treated with antibiotics. The researchers unexpectedly found that soil bacteria carrying antibiotic-resistant genes became more abundant when they were grown with the manure than when they were grown with synthetic nitrogen-based fertilizer—even though the cows were drug-free. The team published its work in October in the Proceedings of the National Academy of Sciences USA.

Previous research has found that manure from pigs treated with antibiotics contains resistant bacteria, including Escherichia coli, but the cow-pie results suggest there are more factors promoting resistance besides antibiotic use. Something about manure itself may encourage naturally resistant bacteria to proliferate.

The findings should not, however, give the perception that resistance is everywhere, notes Lance Price, a microbiologist at George Washington University (who was not involved in the study). Widespread resistance is not inevitable, he says. “We can control this. There's very clear evidence that when we turn off the antibiotic spigot, we bring down drug-resistant bacteria.”

Next on the farm-to-table agenda, Handelsman will test whether radishes grown in soil treated with cow manure are capable of taking up resistant genes from bacteria via their vascular system. “They have veins just like us,” she says. “We don't have any evidence yet that they're taking up the bacteria, but it's a really interesting possibility.”