AILSA CHANG, HOST:
One of the longest running battles on Earth has been ongoing for millennia in soil. Dianne Newman is a microbiologist at Caltech.
DIANNE NEWMAN: In nature, organisms are duking it out. It's a competitive environment. And so one of the strategies that microbes have evolved in order to effectively compete is to produce antibiotics to kill their neighbors.
CHANG: Now, this battle is good for people. The antibiotics that we use ultimately trace back to the soil. But new research suggests that the soil may be a source of resistance to antibiotics. NPR's Jonathan Lambert has more.
JONATHAN LAMBERT, BYLINE: Worldwide, many antibiotics are starting to lose their bite. Researchers know that human use and especially overuse over the past 80 years have given bacteria opportunities to develop resistance. But the natural environment of many bacteria are undergoing major shifts due to climate change, Newman says.
NEWMAN: The thing that wasn't known was whether changes in the natural environment might promote the rise of antibiotic resistance.
LAMBERT: One environmental change that's becoming increasingly common, partly from climate change, is drought. Newman had a hunch that soil drying out could literally concentrate the antibiotics bacteria use to wage war. That could expose bacteria to higher doses of antibiotics.
NEWMAN: Anywhere you increase exposure to antibiotics, you will select for microbes that can withstand.
LAMBERT: To test this idea, Newman and her team first looked at soils from around the globe. They found that in a given region, drier soils had more genes for making antibiotics. If soil from a certain spot got wetter, they saw fewer antibiotic-making genes. They found a similar pattern for resistance genes. The drier, the higher.
NEWMAN: We saw that this elevation resistance occurred over a wide variety of antibiotic classes.
LAMBERT: But does this evolutionary arms race going on in the soil mean anything for human infections? Newman thinks so, since bacteria can swap genes with their neighbors, even from a different species. So she looked to see if bacteria from sick people in hospitals had resistance genes from local soils. Some were an exact match. That suggests a recent swap, perhaps through a scrape in the dirt that got infected.
NEWMAN: These pathogens can be encountered in activities as benign as gardening.
LAMBERT: Finally, the researchers looked to see if there was a relationship between the dryness of soil in a given place and the level of antibiotic resistance in the hospitals there. Sure enough, they found that the drier the environment, the more resistance in hospitals.
NEWMAN: It was a really strong correlation, surprisingly strong.
LAMBERT: The study was published in Nature Microbiology. Timothy Ghaly is a microbiologist at Macquarie University in Australia. He was struck by just how big a difference drought seemed to make.
TIMOTHY GHALY: It's already having an impact on health care systems around the world.
LAMBERT: An impact he says could get worse with climate change. Ramanan Laxminarayan is an epidemiologist at Princeton University. He says the study provides good evidence that drought can increase resistance genes in soil in a way that could affect humans. But linking to hospitals?
RAMANAN LAXMINARAYAN: Now, that's a stretch too far, even after controlling for regional income differences, because there are many things that determine resistance in hospitals.
LAMBERT: For instance, health care systems in drought-prone areas might be more fragile. Still, he thinks their paper should expand how researchers think about where resistance comes from. To ensure antibiotics keep working, people may have to pay closer attention to what's going on beneath our feet.
Jonathan Lambert, NPR News.
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