What does dry soil, like this stretch in South Africa, have to do with antimicrobial resistance? A new study offers an unexpected hypothesis: drought can drive higher antibiotic resistance in soil bacteria.
Rodger Bosch/AFP/via Getty Images
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Rodger Bosch/AFP/via Getty Images
The battle between bacteria and antibiotics is one of the most critical on earth.
In this contest, bacteria seem to be gaining the upper hand. Globally, many antibiotics are becoming less effective, with about 1 in 6 human infections tested in laboratories showing resistance. This contributes to over 4 million deaths annually.
It is well-established that human use, particularly overuse, of antibiotics has allowed bacteria to develop resistance. However, both antibiotics and the resistance to them have a history that predates modern medicine, originating from a long-standing natural battle occurring in the soil.
“In nature, organisms are battling each other in a competitive environment,” explains Dianne Newman, a microbiologist at Cal Tech. “Microbes have evolved strategies to compete effectively, including producing antibiotics to eliminate their competitors.”
This evolutionary arms race has persisted for millennia. Its significance for humans became evident only after the discovery of antibiotics in soil and their use for treating infections began in the 1940s. Newman speculated whether changes in the environment of soil, the primary source of antibiotics, might be contributing to increasing resistance.
According to research by Newman and her colleagues published in Nature Microbiology, drought conditions can enhance antibiotic resistance in soil bacteria. This resistance could potentially spread to human infections.
“It’s a remarkable study, demonstrating that drought is already affecting healthcare systems worldwide,” notes Timothy Ghaly, a microbial ecologist at Macquarie University in Australia who was not involved in the research. “As drought becomes more prevalent globally, antimicrobial resistance is likely to increase as well.”
Focusing on Soil
Drought might seem an unlikely factor driving antibiotic resistance. Yet, Newman hypothesized that when soil dries, the antibiotics used by bacteria to compete could become more potent due to evaporation.
“Imagine a vat of liquid containing antibiotics,” explains Newman. “If the liquid evaporates, the antibiotic molecules become more concentrated.”
This concentration could expose bacteria to higher doses. “Increased antibiotic exposure selects for microbes capable of withstanding them,” she adds.
To investigate this, researchers examined soil samples from around the world. They found that drier soils typically contained more genes responsible for producing antibiotics. Longer drought conditions correlated with more antibiotic-producing genes from various bacteria.
“We predicted that desiccation concentrates these antibiotics, leaving only organisms that can withstand them,” Newman remarks. Laboratory experiments confirmed this prediction.
The researchers further analyzed the soil samples for antibiotic resistance genes, finding more of them in drier soils.
“This study presents a novel pathway suggesting drought increases the concentration of resistance genes,” says Ramanan Laxminarayan, an epidemiologist at Princeton University not involved in the research. While this poses a hazard, it doesn’t necessarily become a risk unless these genes enter human pathogens.
The authors provide evidence of such potential, though the extent of the problem remains debated.
Transmission to Humans
Most soil bacteria are not human pathogens. However, bacteria can exchange genes with their neighbors in the soil, a process known as horizontal gene transfer.
“This allows for rapid spread from soil to clinical settings, where it becomes problematic,” Newman explains. Her team identified several resistance genes linked with drought-stricken soils in bacteria samples from hospital patients. One gene was 100% identical, indicating a recent gene swap.
The pathway by which these soil resistance genes reach human pathogens is unclear. A scrape or cut can facilitate such transfer, suggests Newman. “Microbes are everywhere we go.”
If these transfers are not rare, areas with drier soils might have higher rates of antibiotic-resistant infections. Newman and her colleagues analyzed resistance data from hospitals in 116 countries, finding a correlation between resistance and aridity: the drier the soil, the higher the resistance.
Lower-income countries frequently exhibit higher resistance rates due to factors like inadequate sanitation and less effective infection control. Even when examining only high-income countries, where overall resistance was lower, the correlation between resistance and aridity persisted.
“The correlation is compelling,” says Ghaly. “Such strong correlations are rare in biology.”
However, some scientists remain skeptical.
“That may be an overreach,” says Laxminarayan. “Many factors influence hospital resistance, including healthcare system quality in drought-prone areas.” Further research is needed to determine whether drought directly increases resistant infections.
Despite this, Laxminarayan agrees with other aspects of the study.
“It adds to the evidence that resistance levels may be shifting in the environment, unnoticed by us,” he says. “Eventually, this becomes a risk if it transfers to livestock or humans.”
To prevent this, he suggests researchers should pay closer attention to soil dynamics.
