Tiny fragments of plastic are altering the balance of bacteria in the human gut in ways that resemble the microbiome patterns seen in depression and bowel cancer, new research has revealed.
The study, unveiled at the UEG Week 2025 medical conference, suggests that microplastics, particles smaller than 5mm, can subtly but significantly change how gut bacteria behave, raising fresh concerns over the health impacts of daily exposure.
The work, carried out under microONE, a major COMET Module programme led by Austria’s CBmed research centre with international partners, is one of the first to directly explore how microplastics interact with the human gut microbiome. Researchers say the findings add to mounting evidence that these pollutants, found in everything from bottled water to household dust, could be affecting the body in unseen ways.
To test the effects, scientists used stool samples from five healthy volunteers to grow gut microbiome cultures outside the body. These were exposed to five common types of microplastic, including polystyrene and polyethylene, in concentrations similar to typical human exposure, as well as higher doses to see how the microbes reacted under stress.
While overall bacterial numbers stayed steady, the samples exposed to plastic became markedly more acidic, indicating shifts in bacterial metabolism.
The researchers found that these disruptions also affected the chemical by-products the bacteria produced, including changes to valeric acid, lysine and lactic acid levels. Some of these shifts mirrored those previously linked to mental health disorders and colorectal cancer.
Lead author Christian Pacher-Deutsch said the precise mechanisms behind the disruption remain uncertain but several explanations are emerging. “Microplastics may change microbial composition by creating physical or chemical environments that favour certain bacteria,” he said. “For instance, biofilms can form on microplastic surfaces, providing new niches that some microbes colonise more rapidly.”
He added that microplastics may carry chemical substances that directly interfere with bacterial metabolism. “This can lead to changes in acid production, which may serve as a bacterial stress response, unintentionally altering the gut’s pH,” he said. “These shifts could then trigger feedback loops that further affect the balance of the microbiome.”
Pacher-Deutsch said the findings underline how widespread exposure to microplastics could pose unseen risks. “The key takeaway is that microplastics do have an impact on our microbiome,” he said. “While it’s too early to make definitive health claims, the microbiome is central to digestion, immunity and even mental health. Reducing exposure where possible is a sensible precaution.”
The prevalence of microplastics is significant. Researchers at the University of Plymouth recently found that microplastics could even be lurking in in vegetable after discovering particles found in the soil can be absorbed into the edible parts of crops during the growing process.
Nanoplastics measuring as little as one millionth of a centimetre in diameter were found in radishes. And for the first time, experts found that these plastics can enter through the roots and spread through the plant as they are so small.
Lecturer in physiology Dr Nathaniel Clark said: “Plants have a layer within their roots called a Casparian strip, which should act as a form of filter against particles, many of which can be harmful.”
