Climate change could increase hard-to-treat bacterial infections, two studies suggest.
Heat boosted antibiotic resistance among bacteria found in artificially warmed grassland soils, researchers report April 22 in Nature. And as drought strips the soil of moisture, antibiotics in the environment become concentrated in the little water that remains, encouraging the growth of resistant microbes, another team reports in the April Nature Microbiology.
The two studies point to heat and drought driven by climate change as forces behind a rise in antibiotic resistance in natural environments, which could in turn threaten human health.
Antibiotic resistance has long been linked to human misuse or overuse. The risk arises when patients cut treatment short or when physicians mistakenly prescribe the drugs to treat viral infections that antibiotics can’t cure. But “we often forget or even neglect the historical fact that these clinical drugs are not only present in CVS pharmacies,” says Xiaoyu Shan, a microbial ecologist at Caltech.
Many antibiotics have origins in soil microbes, which use the compounds as weapons to interfere with competitors. While drug developers have leveraged these compounds to treat a variety of bacterial infections, resistance to specific antibiotics can give microbes a survival advantage in both the environment and the human body.
Climate change may also boost antibiotic resistance. Previous studies have shown that higher temperatures are associated with a rise in antibiotic-resistant infections. “But we really don’t know the mechanism,” says Jizhong “Joe” Zhou, a microbial ecologist at the University of Oklahoma in Norman.
Over a decade, from 2009 to 2020, Zhou and colleagues artificially warmed grassland plots to 3 degrees Celsius above the surrounding air temperature with infrared lamps. Genetic analyses of soil samples showed that the abundance of antibiotic resistance genes was roughly 25 percent higher among soil microbial communities in heated soils compared with normal plots.
Warming went hand-in-hand with the increase, the team found. As bacteria adapted in ways that helped them thrive in hotter soil, antibiotic resistance developed, too. And other bacteria that could handle the heat — some of which were already resistant to antibiotics — had a leg up. It’s possible that bacteria swapping genes with one another helped that resistance spread throughout the microbial community. Overall, the findings suggest that soil microbes become resistant over time, not because of exposure to an antibiotic, Zhou says, but due to warming itself.
Dwindling water sources, meanwhile, encourage resistance through exposure, Shan and colleagues found. Data collected from cropland and grassland in California, a forest in Switzerland and a wetland in China revealed that soil microbes produce more antibiotics during a drought than under normal conditions. Experiments in lab dishes showed that antibiotics became concentrated in drier soils. The concentrated compounds killed off sensitive bacteria, allowing resistant strains to flourish.
It’s a bit like making rock candy. “You put sugar in a solution, and if you start evaporating the solution, it will concentrate the sugar to the point where you start making rock candy,” says study coauthor Dianne Newman, a bacterial physiologist at Caltech. (Newman is a member of the board of trustees for the Society for Science, an educational nonprofit in Washington, D.C., that publishes Science News.)
Drying soils don’t produce “candy” but do force antibiotics and microbes to inhabit tighter spaces. That close proximity may help antibiotics kill susceptible bacteria, while other microbes become resistant through gene swapping with their neighbors. Those resistant strains could make it into people. Using data from hospitals in 116 countries, the team found higher frequencies of antibiotic-resistant infections in drier locations.
It’s challenging to link what’s happening in soils directly to the clinic, says epidemiologist and economist Ramanan Laxminarayan of One Health Trust in Washington, D.C., who wasn’t involved with either study. That’s because other factors could also explain why drier regions tend to have more antibiotic resistance, such as a lack of health care facilities in rural deserts that can delay care.
Arid environments tend to be dusty, Shan says. Dirt blowing in the wind could carry microbes across the landscape and expose people to antibiotic-resistant pathogens.
Still, the two studies highlight the importance of paying attention to the world around us, Laxminarayan says. “We’re at the mercy of the environment. It isn’t as if we can solve all public health problems just by working within hospitals. We’re going to have to look at the environment as well.”
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