A study of soil microbes showed that drought favors the microorganisms that survive antibiotics. It also found that some of the genes for resistance in soil-dwelling bacteria show up in antibiotic-resistant pathogen samples collected from hospital patients. Because bacteria can easily swap big chunks of genetic information — a process called horizontal gene transfer — any increase in resistance in soil-inhabiting microbes can easily make its way to microbes that infect humans, the study authors said.

“No place is immune,” said Dianne Newman, the study’s senior author and a biologist at Caltech. “If you have a pathogen arise in one part of the world, it very quickly spreads, so this is something of concern regardless of where you live.”

Antibiotic resistance is already a major health problem, with the World Health Organization estimating that antibiotic-resistant pathogens directly caused 1.27 million deaths per year as of 2019 and contributed to another 4.95 million. While antibiotics kill microbes, the drugs people use in the clinic are also derived from microbes (or fungi, such as in the famous case of penicillin). Microbes synthesize antibiotics as part of an evolutionary arms with other microbes, aiming to kill any potential competitors or threats. One of the major battlegrounds for this evolutionary warfare is in soil.

Newman and the new study’s first author, Caltech postdoctoral researcher Xiaoyu Shan, first uncovered a hint that drought could worsen antibiotic resistance in a set of five metagenomics databases that gather soil microbe genetic information from different environments on continents around the world. Some of these databases included samples from the same sites before and after drought.

In every case, the researchers found, antibiotic synthesis genes were more prevalent after a dry period and less prevalent after a drought ended. “You see this in croplands, in grasslands, in forests, in wetlands, in the U.S., in China, in Switzerland,” Newman told Live Science.