Scientists discover that Roundup and other widely used herbicides cause resistance to medically important antibiotics.
A team of researchers from universities in New Zealand and Mexico have discovered that three herbicides (weed killers) widely used in agriculture and in gardens can make disease causing bacteria resistant to antibiotics.
Their paper, published in the online journal MBio, offers a new perspective on the problem of antibiotic resistance, which may help to explain why it has been increasing so rapidly in recent years.
The three herbicides they looked at were glyphosate (Roundup), the world’s most widely used pesticide, dicamba (Kamba) and 2,4-D. These were tested on E. coli and Salmonella bacteria treated with one of five different antibiotics.  E.coli cause more infections that any other type of bacteria.
In most cases even low levels of the herbicides had the effect of inducing antibiotic resistance before the antibiotics had time to kill the bacteria; in a few antibiotic/herbicide combinations they actually made the bacteria more susceptible to the antibiotic, while in other cases they had no impact.
Residues of these herbicides typically found in food were not sufficient to induce resistance, but the researchers have identified a number of situations in homes and on farms where more direct exposure to the herbicides could result in antibiotic resistance in children and pets or farm animals and pollinating insects, such as honeybees, which can need treating with antibiotics to cure bacterial infections.
The principal concern is that doctors treating life-threatening infections often need to get the dose of antibiotics right first time in order to save lives. To do that they base decisions on existing knowledge of bacterial susceptibility to antibiotics, but where the bacteria are simultaneously exposed to antibiotics and one of the herbicides treating according to existing guidelines will frequently be unsuccessful.
Richard Young, SFT Policy Director commented, “This is a very important scientific discovery. The study shows that the use of herbicides in intensive farming may be one of the reasons that antibiotic resistance has been increasing so rapidly in recent years. We will be calling on UK and EU regulators to consider this research carefully and to reassess the safety of all herbicides to include their impact on antibiotic resistance.
In the US there are an estimated 2 million new antibiotic-resistant infections every day and it has to be noted that glyphosate is used extremely widely in the US on genetically engineered Roundup Ready crops of soya, maize and oilseed rape. It has even been found in rainwater.”
Due to the development of superweeds in the US that have become resistant to Roundup, two new generations of GM crops have been developed, one of which is also resistant to 2,4-D and Roundup, the other to dicamba and Roundup. As a result the use of these herbicides is expected to greatly increase in the coming years.
For further information or requests for interview, please contact:
Richard Young, Policy Director, Sustainable Food Trust:
Notes for Editors
 Ciprofloxacin, chloramphenicol, ampicillin, kanamycin and tetracycline
Q&A with Brigitta Kurenbach, Delphine Marjoshi, Carlos F. Amábile-Cuevas, Gayle C. Ferguson, William Godsoe, Paddy Gibson and Jack A. Heinemann
We found that exposure to some very common herbicides can cause bacteria to change their response to antibiotics. They often become antibiotic resistant, but we also saw increased susceptibility or no effect.
The direction or magnitude of the observed effects were not predictable from the bacterial species, antibiotic or herbicide used.
The effects found are relevant wherever people or animals are exposed to herbicides at the range of concentrations achieved where they are applied. This may include, for example, farm animals and pollinators in rural areas and potentially children and pets in urban areas. The effects were detectable only at herbicide concentrations that were above currently allowed residue levels on food.
Which herbicides did you use in your study?
We used commercial formulations of Kamba (dicamba), 2,4-D and Roundup (glyphosate).
Is this the first study to show this?
To our knowledge, yes. Other substances have been shown to change bacteria’s tolerance to antibiotics (e.g. aspirin), but herbicides were never tested.
Why does it matter?
Antibiotic resistance is a problem for human and animal health. New antibiotics are hard to find and can take decades to become available. Effects of chemicals such as herbicides could conflict with measures taken to slow the spread of antibiotic resistance.
There are more individual antibiotics than we could test in our study. However, we tested examples from most major groups of antibiotics.
We only tested 2 bacterial species. Both were common laboratory strains of species that can cause disease in humans. In the future, we hope to test more species. We found evidence of how the response is caused, but there may be other ways that we have not yet found.
In most cases, we saw increased resistance. But in some cases the response was to cause a greater sensitivity to an antibiotic or no change at all. However, we cannot assume that increasing sensitivity will result in a reduction in antibiotic resistant strains.
Have these results been replicated?
Yes, we conducted our own blind replication by sending the bacteria and chemicals to a researcher at a different university without telling her what they were. She used our protocols and confirmed our findings and then became part of the author team.
Photograph: Mike Mozart
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