You may have heard that some pesticides can disrupt the gut microbiome. In this post, Maaike Gerritse shares findings from the SPRINT project that show how pesticides affect this delicate ecosystem. She also describes the first ever evidence that pesticides can change the microbiome in your nose that protects you from disease.
Back in 2022, I explained why our research project, SPRINT, was so keen to explore the links between pesticides and the gut microbiome in my blog: How and why SPRINT are studying microbiomes.
A microbiome is the combination of millions to trillions of bacteria, fungi and other micro-organisms that live together. A healthy microbiome in your gut, with ‘good’ bacteria, helps protect you from diseases including cancer, obesity, coronary heart disease, diabetes and dementia. Not only that, but it helps you digest fibre, provides certain vitamins, and keeps stomach bugs at bay by keeping your gut occupied: no space for ‘bad guys’.
Fast forward to today, we have now completed our microbiome investigations on SPRINT. You will be able to learn more about many of these in forthcoming peer-review journal papers (anticipated in 2026).
For now, I am very pleased to give you a preview of some of our key findings. As these results have not yet been peer-reviewed, do interpret with some caution. We will list journal papers as they are published on our website – keep an eye out for them there.
Which pesticides affect the gut microbiome?
Our researchers investigated the gut microbiome by extracting bacterial DNA from faecal samples. Based on this DNA, we could see which bacteria are present in the gut microbiome.
We also looked at which pesticides enter the body from our diets and environment by analysing the blood, faeces, and urine of hundreds of volunteers.
From this, we could link eight pesticides or classes of pesticide with small changes in the composition of the gut microbiome. These substances were: pyrethroids, glyphosate, metalaxyl, hexachlorobenzene, pirimiphos-methyl, propiconazole, DDT and chlorpyrifos.
‘Composition’ refers to which bacterial species are present in the gut, and how abundant they are relative to other species. Importantly, four of these pesticides (propiconazole, pirimiphos-methyl, metalaxyl, and hexachlorobenzene) had not been linked to the gut microbiome before. This suggests that more pesticides may be connected with changes in the gut microbiome than previously thought.
The effects of higher pesticide levels on the gut microbiome
As part of SPRINT, we asked volunteers to wear silicone wristbands that absorb pesticides from the environment, and not from their diet, to learn more about which chemicals they encounter in everyday life.
We found that people exposed to certain pesticides have a slightly different gut microbiome than people exposed to a very low number or a low concentration of pesticides.
Our test results suggest that mixtures of pesticides may have more impact on the gut microbiome than pesticides that are tested individually. This is an important finding since we are usually exposed to mixtures of pesticide residues rather than single compounds.
Which gut microbes are affected by pesticides?
A higher pesticide burden (i.e. total pesticide numbers and concentrations) was associated with a loss of some bacteria, including beneficial bacteria. For example, Bifidobacterium was reduced with a higher total pesticide burden.
Research shows that Bifidobacteria reduce inflammation and are linked to a lower risk of gastrointestinal diseases, allergies, metabolic disorders and depression.
Bacteroides, a dominant group of bacteria in the gut, was also reduced with higher concentrations of pesticide mixtures. Bacteroides help provide energy for the gut and prevent inflammation.
The population of Akkermansia, a group of bacteria associated with metabolic health and a lower body weight, was also lower at higher concentrations of multiple pesticides.
These findings indicate that pesticide-associated changes in the gut microbiome might have consequences for human health.
The nasal microbiome?
Some of you might know that SPRINT also investigated the microbiome in the nose. It is not only the gut microbiome that plays a role in human health; the nasal microbiome also supports your immune system as well as your ability to smell.
An imbalance in the respiratory microbiome is suspected to be a risk factor in respiratory infections, such as asthma, but also Parkinson’s disease and Alzheimer’s disease.
The effect of pesticides on the nasal microbiome has not previously been studied. The respiratory microbiome has many parts. For example, the microbiome of the nose is very different to the microbiome in your throat, mouth and lungs. Our researchers were the first research team to find out that pesticide exposure can be linked to changes in the nasal microbiome.
We discovered an association between the total number of pesticides detected on silicone wristbands worn for one week and changes in nasal microbiome composition – as detected by swabbing the nostrils of our participants and extracting bacterial DNA from the swab.
The pesticides on wristbands mostly came from the participants’ environments, rather than their diets. This suggests that the effects on the nasal microbiome may be due to inhalation of pesticides in the air or dust.
Could pesticides affect your mental health and behaviour?
The gut microbiome is increasingly recognised as an important influence on brain function and behaviour, with good gut health linked to improved mental wellbeing.
However, exposure to pesticides, and other chemicals in the environment may influence the relationship between the gut and the brain by altering the presence or balance of microbes that are involved with communications between the two organs.
In SPRINT, we investigated the effects of glyphosate — a commonly used herbicide — on the gut microbiome and behaviour in mice
Normally, mice are curious to meet newcomers. This is even the case for mice kept in a laboratory. We found that male mice had disrupted social behaviour when we exposed them to glyphosate at low levels, i.e., at the ‘acceptable daily intake’ (ADI) which is the limit set by regulators.
Specifically, the mice interacted less with a new mouse that was introduced to the lab setting, compared with mice in the experiment that were not exposed to glyphosate.
In another test, male mice showed some signs of increased anxiety-like behaviour following glyphosate exposure. Female mice showed fewer behavioural changes (to find out more about sex differences, look out for our upcoming blog on pesticides, sex and gender).
These effects appear to be linked to changes in the gut microbiome. Glyphosate exposure corresponded with subtle but measurable shifts in the composition of gut microbes in both sexes.
When we transferred microbiota from glyphosate-exposed male mice to mice that we did not expose to glyphosate, the recipient mice developed the same social behavioural trait as the glyphosate-exposed mice.
This demonstrates that the microbiota changes were sufficient to drive this effect. Anxiety-like behaviour, however, was not transferable. This suggests that different processes are involved in anxiety and social behaviour.
Many studies, including ours, show that glyphosate is associated with small changes in the human gut microbiome. However, more research is needed to see whether similar effects on behaviour occur in humans. These findings raise important questions about how every day, low-dose exposure to widely used herbicides may influence our behaviour through the gut microbiome.
What do these findings mean for your health?
These changes to microbiomes may have consequences for your health. Regulators have many safety considerations when deciding whether to approve a pesticide for the market. The effects of pesticides on the microbiome might be an interesting new factor for them to consider.
However, while we know that the gut microbiome is very important for health, many other factors can interfere with its composition, such as your age, diet, exercise, sleep and medication use.
If you want to protect your gut microbiome, the most important thing is to eat a fibre-rich diet with many fruits and vegetables and consider buying organic products when you can.
To further support your gut health, you should limit your sugar consumption, stay well‑hydrated, get enough sleep, and exercise regularly.
What about your nose? Just as for the gut microbiome, there are many factors that influence the nasal microbiome, and many more studies are needed to better understand how it works.
However, it is possible that regularly cleaning and airing your home may help protect the nasal microbiome from pesticides in your environment. SPRINT has also shown that pesticide residues accumulate in household dust. In our silicone wristband study, we found that study participants who reported regularly cleaning their homes and opening their windows were less likely to have direct contact with pesticides.
Want to know more?
Keep a close eye on our publications page for new research papers on pesticides and the microbiome.
Webinar: Pesticides and Human Gut Health, presented by Maaike Gerritse and Milla Brandão Gois.
Research paper: Pesticide exposure and the microbiota-gut-brain axis. By: Matsuzaki, R., Gunnigle, E., Geissen, V., Clarke, G., Nagpal, J., Cryan, J F., in The ISME Journal (2023).
Research summary: Linking synthetic pesticide exposure to the gut microbiota and brain functioning.
Conference presentation: Pesticide exposure and the microbiota-gut-brain axis, presented by Eoin Gunnigle.
Thank you to Eoin Gunnigle and Paul Scheepers for their valuable contributions to this blog.
