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More than a hundred chemicals in pesticides, plastics and other products that people are routinely exposed to stifle the growth of health-promoting gut bacteria, according to a new study.

Health experts in recent years have increasingly focused on the importance of the gut microbiome and what might harm it. Humans have, on average, hundreds of types of gut bacteria that are critical for digestion, managing weight, immune system function and mental health. These bacteria, along with other types of microorganisms such as viruses and fungi, form what’s referred to as the gut microbiome. Gut bacteria disruptions have been linked to a variety of health problems including allergies, Parkinson’s disease and type 2 diabetes.  

The new study, which was published in Nature Microbiology and builds on previous evidence that certain pesticides and chemicals impact gut health, cautions that most current chemical safety testing does not account for impacts to gut health even though it’s likely people are regularly exposed to many of the chemicals they tested. 

“Certain industrial and agricultural chemicals can inhibit the growth of gut bacteria and select for increased antibiotic resistance,” said Indra Roux, lead author of the study and a researcher in the University of Cambridge’s Medical Research Council Toxicology Unit. “This could disrupt the normal function of the microbiome and reduce the effectiveness of antibiotics.”

Roux and colleagues in lab experiments examined the impact of 1,076 chemicals — including 829 pesticides — on 22 gut bacteria species and found that 168 contaminants were toxic to gut bacteria. The majority of those that stifled bacterial growth were not previously thought to be antibacterial, and the contaminants included several popular herbicides, fungicides, flame retardants and plastic additives. Fungicides and industrial chemicals were most likely to disrupt the gut bacteria.

“Certain industrial and agricultural chemicals can inhibit the growth of gut bacteria and select for increased antibiotic resistance.” – Indra Roux, University of Cambridge

Roux said it’s not entirely clear how the contaminants stifle the bacteria, but they did investigate how some gut bacteria survive the stress caused by pollutants and the experiments showed that “some mechanisms of pollutant resistance overlap with antibiotic resistance.” 

“If similar effects take place in the human gut, they could make infections harder to treat,” she added. 

Some common contaminants affected more gut bacteria than others. For example, the flame retardant tetrabromobisphenol A (TBBPA) and closantel, an antiparasitic used in cattle, both inhibited the growth in 19 strains of bacteria. 

The fungicide and antibacterial compound hexachlorophene topped the list by stifling 20 strains of bacteria. Hexachlorophene use is already restricted by the US Food and Drug Administration (FDA) over health concerns, and the US Environmental Protection Agency revoked all food tolerances for the chemical in 1995.  

Researchers have previously warned of certain pesticides’ impact on gut health. For example, another study released this month found the world’s most widely used weed killer, glyphosate — the active ingredient in Roundup, which is the focus of thousands of lawsuits over cancer claims — disrupts gut bacteria even at tiny exposure levels in lab studies. It is just the latest study to link the weed killer to gut health disruptions, with previous research suggesting glyphosate’s effects on the gut microbiome were caused by the same mechanism by which the weed killer acts to kill weeds and other plants. And there’s evidence that the chemical increasingly used to replace glyphosate, diquat, has similar effects. 

Germany-based Bayer, which bought glyphosate manufacturer Monsanto, has pushed back against studies linking its weedkiller to altered gut bacteria, writing in a July 2024 report that “these studies are not representative of real conditions in the gastrointestinal tract.” The new study included glyphosate but did not find significant impacts. 

The researchers used the same concentration of each chemical so the findings do not necessarily translate to human exposure, Roux said, adding that the work could, however, inform which compounds and effects to focus on in future studies.  

In addition to pesticides and industrial chemicals, researchers have found certain food additives such as preservatives, emulsifiers and artificial sweeteners can disrupt the gut microbiome. 

The authors of the new study say that regulators are not adequately testing chemicals for impacts on gut health. They used their data to create a prototype machine learning model to predict if chemicals will harm gut bacteria, and published it along with the new study.

Senior author Kiran Patil, a professor and researcher in the University of Cambridge’s Medical Research Council Toxicology Unit, said while the new model could eventually help design safer chemicals, it’s important that future research collects real-world exposure data to see if chemicals are affecting human guts similarly to what they observed in the lab. 

Featured image: Kiran Patil, a professor and researcher in the University of Cambridge’s Medical Research Council Toxicology Unit, in the lab. (Credit Jonathan Settle/University of Cambridge)