Toxins don’t stay still. They move. Covertly, mostly; overtly, rarely. Over the decades they have found a home in virtually every environmental niche, wreaking havoc on their surroundings. Their reach is all-encompassing and we are still not fully aware of their impact.
Human activities continue to pump planet-heating carbon dioxide into the atmosphere. The latest global average of carbon dioxide stands at 426.34 parts per million (ppm), much higher than the safe levels of 350 ppm. Atmospheric carbon dioxide is now 50 per cent higher than it was before the Industrial Revolution. We experience the effects day in and day out in extreme weather; in heat, in floods and in illnesses.
No less damaging—perhaps more intimate and immediate—is industrial chemical pollution and toxicity. Nothing—literally nothing—is beyond its encroachment and invasion. Your surroundings, your body and its tissues and organs, its nooks and crannies.
More than 150 common industrial chemicals, in pesticides to flame retardants, that people are routinely exposed to inhibit the growth of bacteria found in a healthy human gut microbiome, according to a study led by University of Cambridge researchers and published in the journal Nature Microbiology. Many industrial chemicals that were thought to be not acting on living organisms do, in fact, act on them.
Gut microbiome—the collection of microorganisms residing in the human gut—has, in recent years, gained a lot of attention as researchers study its utility and what might disrupt it. The human gut is home to about 4,500 bacterial species that help critical functions in the body. They help with digestion, immunity, body weight and mental health. A damaged microbiome can precipitate a variety of health problems including type 2 diabetes, Parkinson’s disease and different allergies.
The researchers, in their analysis of synthetic chemicals, tested 1,076 pollutants found in pesticides and industrial chemicals on the growth of 22 human gut bacteria. They found that 168 were harmful to the bacteria.
“We show here that certain industrial and agricultural chemicals can inhibit or reduce the growth of common gut bacteria. Some pollutants showed a broad range of activity, inhibiting multiple species, while others were more specific to some taxa,” Indra Roux, lead author of the study and a researcher in the University of Cambridge’s Medical Research Council Toxicology Unit, tells Hot Rock.
Revealing the extent of contaminant-gut bacteria interactions, they say “fungicides and industrial chemicals showed the highest proportion of inhibitory activity, with at least one in five compounds suppressing one or more bacterial strains. Notably, around 150 chemical bacteria interactions exhibited strong anti-gut bacterial activity with more than 90 per cent decrease in bacterial growth.” A total of 588 inhibitory interactions were observed involving 168 pollutants.
Since they tested at only a single concentration of 20 micromolars to enable large-screen comparison with previous datasets, Roux says while these results clearly highlight compound classes with antibacterial activity like fungicides, the testing at a single concentration limits the assessment of hazard and risk levels for individual chemicals.
“Our results therefore currently cannot be directly translated to exposure effects on the gut microbiota in vivo, but can inform compounds and effects to monitor in future studies.”
Their experiments showed that some mechanisms of pollutant resistance overlap with antibiotic resistance. For example, when genetic variants of the gut bacteria Parabacteroides merdae became resistant to closantel, they also became resistant to the common antibiotic ciprofloxacin.
“If similar effects take place in the human gut, they could make infections harder to treat,” Roux says.
The findings call for monitoring these effects carefully in future microbiome studies to assess the dynamics of antibiotic resistance genes to make informed decisions. Previous research already shows overuse of medications affect the gut microbiome and can lead to antimicrobial resistance.
Roux says similar assessments should be performed on exposure to pollutant chemicals. The present analysis sprang from the team’s earlier work, where they showed that “non-antibiotic human-targeted medications can unexpectedly act like antibiotics against gut bacteria.” That discovery raised the question: what other chemicals might be influencing our microbes?
During the COVID lockdown, Roux recollects, PhD student Anna Lindell, a co-author on the present paper, began investigating what was already known about how pollutants affect the gut microbiome. That highlighted that a microbiome might encounter more than 25,000 chemicals, spanning food compounds, medications, pollutants and others.
The review says: “The research to date also focuses mainly on antibiotics and host-targeted drugs, leaving other xenobiotics, such as environmental pollutants, underrepresented.”
Therefore, it was useful to learn that environmental contaminants deserve further study: a systematic effort to map how different pollutant chemicals interact with gut bacteria, Roux says.
Going forward, they envisage that “our pollutant-bacteria interaction data will encourage collection of exposure data in microbiome studies.” The study underscores the need for collecting real-world exposure data from people living in polluted areas with high occupational exposure. That helps in assessing the impact on bodies. Roux says microbiome toxicity should be incorporated into safety assessments.
“It would be useful to include testing the effect of chemicals on human gut bacteria as part of approval processes for new chemicals and products. This will be a relatively inexpensive testing with potential population-scale benefits.”
Another takeaway from their study is that the experimental data aligned with machine learning-based predictions of chemical toxicity. It could help in designing new chemicals that are safe.
“It shows that toxicology machine-learning models could be used to guide the development of “safe-by-design” chemicals in the future,” Roux says.
