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, the world’s most widely used pesticide (formulations are sold by Monsanto as ‘Roundup’), dicamba (Kamba), which is proprietary to Monsanto, and 2,4-D, the active ingredient of the notorious ‘agent orange’ herbicide used by the US military to ‘defoliate’ rainforests in Vietnam and Cambodia in the 1960s.

These were tested on E. coli and Salmonella bacteria treated with one of five different antibiotics: Ciprofloxacin, chloramphenicol, ampicillin, kanamycin and tetracycline. E.coli cause more infections that any other type of bacteria. Both E.coli and Salmonella can cause serious, even fatal, infections.

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.

The danger is on-farm, not in food

Residues of these herbicides typically found in food were not sufficient to induce antibiotic 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.

“The effects found are relevant wherever people or animals are exposed to herbicides at the range of concentrations achieved where they are applied”, say the authors.

“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.”

Those at risk of suffering the immediate consequences also include farm workers and others exposed to the herbicides while applying them, residents close to farms within range of ‘spray drift’, and 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, doctors may prescribe antibiotic doses too low to control the infections, creating the opportunity for resistance to those antibiotics to develop.

And then, anyone infected by the resistant antibiotics would be at risk.

Roundup to blame for rising observed antibiotic resistance?

“This is a very important scientific discovery”, said Richard Young, Policy Director with the Sustainable Food Trust. “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 [canola]. 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 ‘stacked’ GM crops have been developed which are resistant to more than one herbicide.

Dow has engineered its ‘Enlist’ varieties of soybean and corn varieties that are engineered to resist its ‘Enlist Duo’ herbicide, a mix of glyphosate and 2,4-D. Both the crops and the herbicide were approved in 2014 by the US Department of Agriculture (USDA). Monsanto has also applied for USDA clearance to sell GMO soybean and cotton varieties engineered to resist both Roundup and dicamba.

The use of these varieties appears certain to further increase the volumes used of the three herbicides examined in the study, giving a further boost to antibiotic resistance.

Another possibility – not tested in the recent experiment – is that certain combinations of the three herbicides may have even greater effects in stimulating antibiotic resistance than when they are acting singly. There are also many other combinations of potentially pathogenic bacteria, herbicides (and other pesticides) and antibiotics that are yet to be tested.

And as the authors warn: “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.”

The paper: ‘Sublethal Exposure to Commercial Formulations of the Herbicides Dicamba, 2,4-Dichlorophenoxyacetic Acid, and Glyphosate Cause Changes in Antibiotic Susceptibility in Escherichia coli and Salmonella enterica serovar Typhimurium‘, by Brigitta Kurenbach, Delphine Marjoshi, Carlos F. Amábile-Cuevas, Gayle C. Ferguson, William Godsoe, Paddy Gibson, & Jack A. Heinemann, is published by mBio.

Principal source: Sustainable Food Trust.

Also on The Ecologist: ‘IARC: Glyphosate probably carcinogenic‘: The International Agency for Research on Cancer has labelled the world’s biggest herbicide, glyphosate, as ‘probably carcinogenic’. Another four widely used organophosphate insecticides may also pose a cancer risk.