Crop Comments: Weedkiller Helps Select for Antibiotic Resistance

In March 2015, the International Agency for Research on Cancer (part of the World Health Organization) pronounced judgment against the herbicide ingredient glyphosate. IARC said that glyphosate “probably causes cancer in humans.” Most of the pillars in the ag chemical community opposed that declaration. An industry spokesperson was chosen to publicly defend that chemical’s safety in a TV interview with investigative reporter Paul Moreira. This herbicide’s enthusiast was given the chance to prove the safety of this ingredient, after he said that glyphosate was harmless to humans. He said a person could drink a quart of glyphosate without harm; he then offered to do just that. However, he immediately reneged on that offer when Moreira called his bluff by actually offering him a glass of the liquid herbicide.

In March 2026, I discovered an article titled “Use of controversial weedkiller inadvertently selects for drug-resistant bacteria that can spread to hospitals.”

The lead scientist, Daniela Centrón, Ph.D., stated that in October 2025, the WHO again sounded the alarm on the emergence of multidrug-resistant bacteria in hospitals around the world. (Centrón, a researcher at the Institute of Medical Microbiology & Parasitology in Buenos Aires, senior-authored the study which is summarized in the above article.) Researchers found evidence that the use of weedkillers, particularly glyphosate, drives the evolution of antimicrobial resistance in soil bacteria as a side-effect of developing resistance to the weedkiller itself. Centrón and her team theorized that resistant bacteria can be transmitted between hospitals and impacted soils in both directions through wastewater and other environmental pathways.

Glyphosate fosters antibiotic resistance by acting as a stressor that selects for bacteria with multi-drug efflux pumps, enhancing the horizontal transfer of resistance genes between bacteria, altering microbial community composition. (Expressed simply, an efflux pump is a physiological trait of a bacterium that serves as a line of defense, which exports toxins (like antibiotics) out of their cells.)

While not directly mutagenic (gene altering), efflux’s widespread use exerts selective pressure, encouraging the survival of bacteria that are resistant to both the herbicide and conventional antibiotics. Each year, antimicrobial resistance (AMR) is responsible for millions of deaths worldwide. Scientists found evidence that the spread of AMR isn’t always driven by bacteria evolving to resist the antibiotics themselves; certain weedkillers can have the same effect.

Quoting Centrón: “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.”

In 2018 and 2020, Centrón and colleagues had collected 68 bacterial strains from sediments in a nature reserve in the Paraná delta, an important wetland located north of Buenos Aires. Glyphosate is frequently applied to nearby agricultural areas. The scientists tested each strain’s degree of resistance to 16 common antibiotics (such as ampicillin combined with tetracycline).

They also measured the strains’ resistance to pure glyphosate and glyphosate-based herbicides – chosen because they are among the most frequently used herbicides globally. The researchers compared the results with those from 19 strains, including multidrug-resis- tant species, sampled from local hospitals. Another 15 strains had been isolated from feedlots and herbicide-impacted ag soils in the region.

As expected, the hospital strains were each resistant to between one and 16 of the antibiotics tested, confirming widespread AMR. Worryingly, 74% were resistant to carbapenems, broad-spectrum antibiotics commonly used as a treatment of last resort. Importantly, all hospital strains also proved 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,” said author Dr. Camila Knecht. “Strains from the Paraná delta spanned 15 genera, including Acinetobacter, Pseudomonas, Exiguobacterium and Chryseobacterium.”

Each had at least partial resistance to glyphosate and glyphosate-based weedkillers, even though these have never been used in the reserve itself.

Enterobacter strains tolerated the highest concentrations of glyphosate (up to 80 milligrams/milliliter). At the other extreme, Bacillus strains, usually found in soils, were particularly susceptible: their growth was already inhibited at a concentration of 2.5 mg of glyphosate per mL. High resistance to glyphosate was also found in strains isolated from hospital infections with extreme drug resistance.

When the scientists made a “family tree” of all 102 bacterial strains, those most resistant to glyphosate tended to be close relatives, regardless of their location of origin. For example, the same genera were found to be resistant against glyphosate across hospitals, agricultural areas and the Paraná delta.

According to Dr. Jochen A. Müller, a group leader at Karlsruhe (Germany) Institute of Technology, “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.”

Glyphosate use abounds with controversy: it is known to harm arthropods (particularly bees). Add that to the judgment passed by the IARC 11 years ago on the herbicide’s carcinogenic traits. For this reason, France, Belgium and the Netherlands have banned glyphosate for household use, while Germany currently prohibits its use in public spaces.

Here’s a rather in-your-face quote from Centrón: “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 warning that genes for antibiotic resistance can spread from glyphosate-contaminated soils to hospitals through untreated water.”

I’m still grappling with the idea of overused herbicides spawning antibiotic resistance in microbes. But not that long ago I wouldn’t have believed that Palmer’s amaranth, a renegade pigweed, could ever develop glyphosate resistance.

by Paris Reidhead