Microplastics found to change gut microbiome in first human-sample study

New research presented today at UEG Week 2025 shows that microplastics – plastic particles smaller than 5mm commonly found in the environment – can alter the human gut microbiome, with some changes resembling patterns linked to depression and colorectal cancer.¹

This study, conducted within the framework of microONE, a pioneering COMET Module programme project led by CBmed research center in collaboration with international partners, is among the first to directly examine how different types of microplastics interact with the human gut microbiome. As part of microONE’s broader mission to explore the effects of micro- and nanoplastic particles in the human body, it offers important new insights into potential health impacts and highlights the urgent need for further investigation.

The study used stool samples from five healthy volunteers to grow ex vivo gut microbiome cultures. These cultures were then exposed to five common microplastic types – polystyrene, polypropylene, low-density polyethylene, poly(methyl methacrylate) and polyethylene terephthalate – at concentrations reflecting estimated human exposure, as well as higher doses to investigate potential dose-dependent effects.

While total and viable bacterial cell counts remained largely unchanged, microplastic-treated cultures showed a consistent and significant increase in acidity (lower pH levels) compared to controls, indicating altered microbial metabolic activity.

Further analysis revealed microplastic-specific shifts in bacterial composition, with certain bacterial groups increasing or decreasing depending on the microplastic type. Changes were observed across several bacterial families, including Lachnospiraceae, Oscillospiraceae, Enterobacteriaceae and Ruminococcaceae, with the majority occurring within the phylum Bacillota – a key group of gut bacteria important for digestion and overall gut health.

These shifts in bacterial composition were accompanied by changes in the chemicals produced by the bacteria, some of which corresponded with the observed decreases in pH. Certain microplastic types altered levels of valeric acid and 5-aminopentanoic acid, while others affected lysine or lactic acid, highlighting the complexity of microplastic-microbiome interactions.

Importantly, some of these microplastic-induced changes in microbial composition reflected patterns previously linked to diseases such as depression and colorectal cancer^2-5, underscoring the potential implications of microplastic exposure for disease risk.

Commenting on the underlying mechanisms behind this effect, lead author Christian Pacher-Deutsch explained, “At this stage, the exact pathways remain unclear, but several plausible explanations are emerging. Microplastics may change microbial composition by creating physical or chemical environments that favour certain bacteria. For instance, biofilms can form on microplastic surfaces, providing new niches that some microbes colonise more rapidly.”

He added, “Microplastics may also carry chemical substances that directly influence bacterial metabolism. This can lead to changes in acid production, which may serve as a bacterial stress response, unintentionally altering the gut’s pH. These shifts could then trigger feedback loops that further affect the balance of the microbiome.”

Reflecting on the broader implications of the study, Pacher-Deutsch said, “These findings are significant given how pervasive microplastic exposure is in everyday life. Microplastics have been found in fish, salt, bottled water, and even tap water, meaning that most people are exposed daily through ingestion, inhalation and skin contact.”

“The key takeaway is that microplastics do have an impact on our microbiome. While it’s too early to make definitive health claims, the microbiome plays a central role in many aspects of well-being, from digestion to mental health. Reducing microplastic exposure where possible is therefore a wise and important precaution,” he concluded.