Scientists have found evidence that glyphosate, one of the world's most widely used herbicides, may help select for bacteria that are resistant to multiple antibiotics.

"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," said Dr. Daniela Centrón, a researcher at the Institute of Medical Microbiology and Parasitology in Buenos Aires and the senior author of the study published in Frontiers in Microbiology.

"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."

For decades, Roundup was closely associated with glyphosate, the herbicide first registered in the United States in 1974. Today, the answer is more complicated. Home-use Roundup products found in many hardware and garden stores have been reformulated without glyphosate and may contain ingredients such as triclopyr, fluazifop, and diquat. But glyphosate remains in professional and agricultural Roundup products used in farming, landscaping, and other commercial settings.

Testing Bacteria From Nature, Farms, and Hospitals

To investigate the connection, Centrón and her colleagues analyzed 68 bacterial strains collected in 2018 and 2020 from sediment in a protected nature reserve in the Paraná delta, a wetland region north of Buenos Aires. Although herbicides have never been applied inside the reserve, glyphosate is commonly used in nearby agricultural areas.

The researchers examined how resistant each strain was to 16 commonly used antibiotics, including ampicillin combined with sulbactam, meropenem, tetracycline, and vancomycin. They also tested resistance to pure glyphosate and glyphosate-based herbicides, which are among the most widely used weedkillers worldwide.

The findings were then compared with 19 bacterial strains obtained from local hospitals, including multidrug-resistant species. An additional 15 strains came from feedlots and agricultural soils affected by herbicide use.

Hospital Superbugs Also Resist Glyphosate

The hospital strains showed widespread antimicrobial resistance. Individual strains were resistant to between one and 16 of the antibiotics tested. Particularly concerning was the finding that 74% were resistant to carbapenems, a class of broad-spectrum antibiotics often reserved as a last line of defense against serious infections.

All of the hospital-derived strains were also highly resistant to glyphosate and glyphosate-based herbicides.

"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," said first author Dr. Camila Knecht from Dr. Centrón's research group.

The 68 strains collected from the Paraná delta represented 15 different genera, including Acinetobacter, Pseudomonas, Exiguobacterium, and Chryseobacterium. Every one of them displayed at least some resistance to glyphosate and glyphosate-based herbicides, despite the fact that these chemicals had never been applied within the reserve itself.

Among the environmental strains, Enterobacter species tolerated the highest glyphosate concentrations, surviving levels of up to 80 milligrams per milliliter. In contrast, Bacillus species, which are commonly found in soil, were especially sensitive. Their growth was inhibited at concentrations as low as 2.5 milligrams per milliliter. High glyphosate resistance was also observed in strains isolated from hospital infections that showed extreme drug resistance.

Resistant Bacteria Share Similar Genetic Backgrounds

The researchers then constructed a genetic "family tree" using all 102 bacterial strains included in the study. They found that bacteria with the greatest glyphosate resistance were often closely related, regardless of whether they originated from hospitals, farms, or the Paraná delta.

For example, the same bacterial genera showed glyphosate resistance across all three environments.

"In the environment, the use of glyphosate leads to the evolution of resistant bacteria in impacted soils, whereas the use of antibiotics favors 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," concluded coauthor Dr. Jochen A Müller, a group leader at Karlsruhe Institute of Technology.

Concerns About Glyphosate and Public Health

Glyphosate has long been the subject of scientific and regulatory debate. Research has shown that it can harm arthropods (in particular bees), and the International Agency for Research on Cancer classifies it as a probable human carcinogen.

Several European countries have already restricted some uses of the herbicide. France, Belgium, and the Netherlands have banned glyphosate for household applications, while Germany currently prohibits its use in public spaces.

Based on the findings, the researchers argue that pesticide regulations should take antibiotic resistance into account before products reach the market.

"Policies for the use of any pesticide, as well as its metabolites, should stipulate the requirement for co-selection testing with antibiotics before marketing. Labels should include a warming that genes for antibiotic resistance can spread from glyphosate-contaminated soils to hospitals through untreated water," counseled Centrón.