New study exposes glyphosate’s link to antibiotic resistance

March 24, 2015 by Staff Reporter

The presumed ‘safety’ of glyphosate, the world’s most widely used herbicide, has been challenged again – this time with evidence linking it to potentially deadly antibiotic resistance.

Late last week the International Agency for Research on Cancer (IARC) a branch of the World Health Organization, declared the herbicide, which is used widely on GMO crops, “a probable human carcinogen.”

Now, an international research team from New Zealand and Mexico, writing in the American Society of Microbiology’s journal mBio has shown that glyphosate (the active ingredient in the product Roundup) and two other widely-used herbicides, 2,4-D and dicamba, change the way bacteria responded to a number of antibiotics, including ampicillin, ciprofloxacin, and tetracycline which are widely used to treat a range of serious, and sometimes deadly, diseases.

‘Non-target’ effects

It has long been known that pesticides can have harmful effects outside the parameters of their intended use (which is to kill a weed or an insect or other microorganism). They can kill non-target insects like honey bees, or non-target plant species like the milk weed – as they have in the US thus depriving the Monarch butterfly of its main food source and placing that iconic insect on the endangered list.

Many pesticides can also be toxic in a number of different ways which defy the notion that the ‘dose makes the poison’. High exposures can act as acute poisons, but very low exposures can have a different effect, for instance acting like hormone disrupters. Evidence of this kind of action is the likely reason why glyphosate has been declared a probable human carcinogen. Most of the possible adverse effects of pesticides are simply not studied before they are put on the market.

Sub-lethal effects

As Jack Heinemann, professor of genetics at the University of Canterbury in New Zealand, and the new study’s lead author 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.

Heinemann and colleagues’ study is different because it looks at sub-lethal effects – in other words what happens when the bacteria are not killed outright but instead remain alive.

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.

When they exposed two bacteria strains – E. coli and Salmonella – to the herbicides they found they activated proteins known as efflux pumps which bacteria use to get rid of toxins. This defence mechanism helps the bacteria develop resistance to a toxic substance.

Last year another study from Brazil found that glyphosate also encourages resistance in Pseudomonas aeruginosa a bacteria that causes infection, especially in patients with compromised immune systems.

Agricultural communities at particular risk

“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% of the soybeans and 89% of the corn grown in the US, while 2,4-D is the third-most widely used herbicide in the US, while dicamba ranks fifth in use worldwide.

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

His team’s results suggest there is probably a small chance that exposure through food would produce these effects, but they could be a particular concern in areas where the pesticides are being applied making them a threat to the health of farmers, farmworkers, and other people who live in agricultural communities.

How this links to the GMO debate

Glyphosate is everywhere and not just in food crops. It is sprayed on roadsides, in parks and on pavements. It is also sprayed on non-GM grain crops as a desiccant (to dry them out) before harvest.  Perhaps not surprisingly, recent surveys of bread in the UK have found that 60% of loaves contain residues, with glyphosate being the pesticide most frequently detected.)

But without a doubt it is the introduction of glyphosate tolerant (Roundup Ready) GM crops that has driver its global use up.

The majority of GM crops are bred to be resistant to glyphosate and there is evidence that not only is glyphosate more heavily sprayed on GM crops (because surrounding weeds have quickly developed resistance to it), but that these plants contain many times more glyphosate, and its toxic breakdown product AMPA, than normal crops. This residue is passed on into the food chain.

To deal with this growing problem of resistance in weeds, last year the US approved two new types of GMO crops – resistant to 2,4-D and dicamba. This is expected to vastly increase the amount of these toxic substances in agricultural use.

Antibiotic resistance is a serious and growing problem for human and animal health. New antibiotics are hard to find and can take decades to become available. The untested effects of chemicals like herbicides, especially as GMO crops increase their use, could make tackling the spread of antibiotic resistance that much more difficult.