Study Links Widely Used Pesticides to Antibiotic Resistance | Civil Eats

Study Links Widely Used Pesticides to Antibiotic Resistance

Glyphosate, 2,4-D, and dicamba found to affect bacteria in ways that could promote resistance to common antibiotics.

This has not been a good week for glyphosate, the active ingredient in Roundup and other herbicides. On Friday, the World Health Organization (WHO) announced that it had classified glyphosate, the United States’ most widely-used pesticide, as “probably carcinogenic to humans.”

Now, the chemical has another strike against it. A study published today by the American Society of Microbiology’s journal mBio has linked glyphosate and two other widely-used herbicides–2,4-D and dicamba–to one of the most pressing public health crises of our time: antibiotic resistance.

This study found that exposure to these herbicides in their commercial forms changed the way bacteria responded to a number of antibiotics, including ampicillin, ciprofloxacin, and tetracycline–drugs widely used to treat a range of deadly diseases.

Dicamba, 2,4-D, and glyphosate have been in use for decades, so why have their antibacterial-resistance effects not been documented before? As the study’s lead author, Jack Heinemann, professor of genetics at the University of Canterbury in New Zealand, explains, when pesticides are tested for adverse effects, “it’s the lethal toxicity that people focus on.” In other words, how much of the chemical will kill an organism.

“What makes our study different, is that it is looking at a sub-lethal effect,” says Heinemann. “The effect we see requires that the bacteria stay alive.”

Previous studies done by other researchers have found that substances chemically similar to dicamba and 2,4-D can cause antibiotic resistance, Heinemann explains. So he and his colleagues decided to investigate whether these herbicides would produce similar effects. They added glyphosate to the study because it is chemically unlike the other two. But, to their surprise, it also produced some antibiotic resistance.

Heinemann explains that because these herbicides are not “supertoxic” to the bacteria the study tested–E. coli and Salmonella–they are not killed outright at levels typically used to kill weeds. Instead, the bacteria stay alive while activating proteins known as efflux pumps in order to rid themselves of toxins. And this defense mechanism can make the bacteria develop resistance to the threat from which it is defending itself.

Scientists know that overuse of antibiotics in humans can decrease their effectiveness. In the same way, says Heinemann, “exposure to these pesticides make the pathogens stronger.”

Although this study only looked at two laboratory strains of human pathogens, the antibiotics examined represent what he calls “broad classes” of drugs we’ve come to depend on to fight infections and the herbicides are three of the most-used worldwide.

Heinemann also notes that the different pesticides produced a variety of responses. While all three produced an antibacterial-resistant response to some of the antibiotics, some of the combinations his team tested produced no response and some increased the antibiotic’s effect.

Although the study is likely to be seen as controversial by some, University of Massachusetts Dartmouth assistant professor of biology, Dr. Mark Silby says it “followed established protocols” and the existing scientific literature supports its findings.

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“This is a very carefully-designed study,” says Dr. Michael Hansen, a senior staff scientist at Consumers Union. “It’s incredibly important work showing the complexity of an effect that hadn’t been thought about before.” The mechanisms by which the bacteria respond to toxics–in this case herbicides–are already well-known, Hansen explains. What’s new and important is looking at non-lethal levels of exposure in combination with the antibiotics.

The weed-killers used in the study were purchased at a local store and were used at levels specified in use directions, which means the scientists were testing chemicals actually in use worldwide rather than a special laboratory sample of the active compound.

How could any of this affect people?

“These herbicides are now used at such a scale that we can almost use the term ubiquitous,” says Heinemann. For one, glyphosate is used on about 94 percent of the soybeans and 89 percent of the corn grown in the U.S, while 2,4-D is the third-most widely used herbicide in the U.S., while dicamba ranks fifth in use worldwide.

The levels at which the researchers saw effects were higher than the residues allowed on food, but below what is often used in rural settings, says Heinemann.

The results of Heinemann’s study suggest there is probably a small chance that exposure through food would produce these effects, but they could be a concern in areas where the pesticides are being applied, says Hansen. Thus, the people most likely to be affected are farmers, farmworkers, and other people who live in agricultural communities.

Also to consider is the approval earlier this year of a new pesticide that combines glyphosate and 2,4-D and soybean and cotton seeds genetically engineered to resist dicamba, all of which are expected to increase use of these pesticides.

Pesticide-induced antibiotic resistance could also affect honeybees since many commercial hives are now being treated with antibiotics. It’s possible, Heinemann says, that “comingling of antibiotics and herbicides could be compromising the effectiveness of those antibiotics,” and thus honeybee health.

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Meanwhile, Monsanto says it disagrees with WHO’s announcement on glyphosate. “All labeled uses of glyphosate are safe for human health and supported by one of the most extensive worldwide human health databases ever compiled on an agricultural product,” the company says in a statement on its website.

Neither Monsanto nor other pesticide manufacturers have had the opportunity to respond to the new mBio study. But the Council for Biotechnology Information said on its website “GMO Answers” last month, that glyphosate had once been considered for use as an antibiotic but that “levels needed to kill microbes are relatively high, and resistance can develop readily.” In other words, the phenomenon Heinemann and colleagues observed is not entirely unexpected.

“A jigsaw puzzle is a good metaphor,” for how these effects fit together, says the scientist.

The next steps in this research will be to test additional bacteria and pure samples of the pesticides. But for now, it’s clear that “further work is needed,” says Hansen. “This is something we need to look at as we expand the use of these herbicides.”

Elizabeth Grossman was a senior reporter for Civil Eats from 2014 to 2017, where she focused on environmental and science issues. She is the author of Chasing Molecules, High Tech Trash, Watershed and other books. Her work appeared in a variety of publications, including National Geographic News, The Guardian, The Intercept, Scientific American, Environmental Health Perspectives, Yale e360, Ensia, High Country News, The Washington Post, Salon, The Nation, and Mother Jones. She passed away in July 2017, leaving behind a legacy of dedication to her mission of journalism that supports and protects people and the planet. Read more >

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  1. Mary Neal
    It's always "further work is needed". In the meantime our civilization is being made the guinea pig and we are one step closer to extinction. But, not to worry, as long as chemical companies make their billions on the backs of the rest of us. Shame...
  2. It is astounding that nobody has asked why are we constantly increasing antibiotic use in feed animals who for years have been fed roundup ready corn and soybean . Now we have a path to follow the cascade. Politicians who have allowed executives from Monsanto, Dow, Cargill to be placed into positions of authority in the EPA, FDA, USDA definitely crossed over with the increase of use in both human and animal consumed foods from GMO seeds clearly reflects conflicts of interests as well as usery and campaign contributions which are bribes. Honey bees produce naturally up to but not limited too 6 different type of antibiotic substances the constitute the bee's wax needed to build a hive. Follow the money. Shame on us if we do not act.
  3. wendy
    It seems as if monsanto is single handedly taking down the entire world killing not only weeds but animals and humans, not to mention the soil and the water. great company.
  4. Kim
    It is my understanding that Roundup is good at killing the "good" bacteria in our intestines leaving the "bad" bacteria to flourish. People with celiac disease and autism often have these gut microflora imbalances and of course two diseases that have been on the rise. I just read that giving antibiotics to some autistic children can lessen their symptoms, I wonder if it is this combination of herbicide/antibiotic that wipes out the bad guy bacteria giving the good bacteria a chance to repopulate.
  5. Jay
    Why does the title of this article say pesticides?
    Dicamba, 2,4-D, and glyphosate are herbicides.
    • Twilight Greenaway
      Pesticide is a general class of chemical that includes herbicides, fungicides, miticides, and others. See:
  6. Scott Rausch
    Considering when Roundup initially was released, it was known to be or have antibiotic properties, it is unbelievable that people routinely use it on animal feed and human food where it does not break down. It is a simple conclusion to expect antibiotic resistance when an antibiotic is fed to either humans or animals that then become human food. The travesty is that Monsanto bought their way to prevent all testing that could show harmful effects of Roundup in the US. Now, all US citizens either are part of the experiment of Roundup, or they hope and pray that a foreign country or group finds out exactly how damaging this product really is.
  7. Renee Gorman
    very interesting, so I guess organic is worth the extra cost then..?
  8. I might be wrong but it sounds to me like Glyphosate could very likely have a role in Anthrax and H1N1.
  9. Thu
    Yes, this is hot issue and we expect that the information will not only the science information that only highly educated people can understand. Further interpretation to what kinds of foods are affected by these chemicals should be disclosed so that people can be well informed. Particularly, as known, Vietnamese are crazy about American imported agricultural products.
    Thank you very much!
  10. This is very disturbing, thank you for this important report.
  11. One of the best findings these are. What I am thinking is; is there any chance of resistance development in insects against pesticides through high use of antibiotics for treatment of animals??

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