Common weedkiller could fuel a rise in superbugs

Antimicrobial resistance, or AMR, occurs when bacteria, viruses, fungi and parasites develop resistance to medicines like antibiotics, which are designed to kill them.

The World Health Organisation (WHO) lists AMR as one of the top threats to global health and development. Each year, AMR is linked to an estimated 1.1 to 1.4 million deaths worldwide. The overuse and misuse of antimicrobial drugs in humans, animals and plants are thought to be the main drivers in the development of drug-resistant pathogens.

But a new study published in Frontiers in Microbiology has revealed that AMR in bacteria isn’t always driven by microbes evolving to resist antibiotics. They found that some weedkillers, such as glyphosate, can have the same effect.

Dr Daniela Centrón, a researcher at the Institute of Medical Microbiology and Parasitology in Buenos Aires and the senior author of the study, says, “Here we show that the most common species of multidrug-resistant bacteria from hospitals are not only resistant to multiple antibiotic classes, but also to high concentrations of the weedkiller glyphosate.”

“These results suggest that weedkillers, which, unlike antibiotics, are widely applied in agricultural environments, may have the unintended side-effect of selecting for AMR among bacterial communities within the soil.”

How is weedkiller linked to antibiotic resistance?

Researchers studied more than 100 different strains of bacteria collected from three environments: a protected wetland in Argentina’s Paraná Delta, nearby farmland where glyphosate had been used and local hospitals.

They tested each strain against 16 common antibiotics as well as pure glyphosate and glyphosate-based herbicides.

As expected, each of the bacteria strains found in hospitals showed resistance to at least one of the antibiotics. But remarkably, all strains were also highly resistant to glyphosate and glyphosate-based weedkillers.

This means that if these bacteria enter the environment through untreated wastewater from hospitals, they could go on to thrive in agricultural areas where glyphosate is used.

Bacteria strains from the Paraná Delta each had at least partial resistance to glyphosate and glyphosate-based weedkillers. This is despite the fact that glyphosate has never been used in the reserve itself.

Scientists mapped the family tree of all 102 bacteria strains examined in this study. They found that those most resistant against glyphosate tend to be close relatives regardless of whether they are found in hospitals, agricultural areas and the Paraná delta.

Dr Jochen A Müller, a group leader at Karlsruhe Institute of Technology and a coauthor of the study, says, “In the environment, the use of glyphosate leads to the evolution of resistant bacteria in impacted soils, whereas the use of antibiotics favours their evolution in hospitals.”

“Bacteria carrying antibiotic resistance genes can spread and breed between those two niches in both directions and in multiple ways, with the water cycle playing a key role in transmission.”

The link between health and the environment

Glyphosate is one of the world’s most widely used herbicides, but it has been coming under increasing amounts of scrutiny. Studies have linked it to harmful effects on insects, in particular bees, and the International Agency for Research on Cancer has classified it as a probable human carcinogen.

Some countries, including France, Belgium and the Netherlands, have banned the use of glyphosate in households, while Germany currently prohibits its use in public spaces.

Researchers argue that regulations should go further in light of this study.

“Policies for the use of any pesticide should stipulate the requirement for co-selection testing with antibiotics before marketing,” says Daniela. “Labels should include a warning that genes for antibiotic resistance can spread from glyphosate-contaminated soils to hospitals through untreated water.”

Although not involved in the study, scientists at the Natural History Museum emphasise that the findings reflect a broader pattern that human health is closely tied to the natural environment.

Dr David Redding, who leads the Museum’s Biodiversity and Health research theme, says, “The relationship between biodiversity and human health is stronger than most people realise.”

“It shapes infectious disease risk, air and water quality, heat exposure and mental wellbeing amongst many other aspects of our health. These are not marginal concerns, they affect population health at a vast scale, and this means our health is sensitive to how we manage the natural world.”

The Museum’s vast collections can give an extraordinary window into how biological systems have changed across the globe over time. David says that combing this with cutting-edge science can help answer questions about the links between environmental change and health outcomes.

“If we can better characterise how biodiversity loss, land use change, and urban design affect health outcomes, for example, we can start to build an evidence base that genuinely informs policy as the world changes.”

“The ambition is for ecological research to sit alongside clinical and social science as a recognised pillar of public health thinking.”