Earth’s oceans contain vortices of plastic waste. These slowly swirling patches of suspended debris span a collective area of millions of square kilometres.[i] Polyethylene, commonly used for medical, cosmetic, food packaging and industrial purposes, is the most prevalent marine plastic contaminant.[ii] Much of this waste is microplastic particles (PM), which upset the delicate balance of marine and terrestrial ecosystems from the tiniest microorganisms up the food chain. Humans consume up to five grams of MPs per week[iii]— the equivalent of five paperclips — through various pathways, turning the human body into its own reservoir of MP waste.[iv] Researchers have found MPs circulating in human blood[v] and islets of MPs in various organs, including the liver, kidneys, spleen, lungs, and placenta. [vi],[vii],[viii],[ix],[x] Humans also excrete MPs in their feces.[xi]
The gastrointestinal (GI) tract is the most common portal of entry for MPs and is a gateway to more widespread systemic dysfunction. In a recent study published in The Hazardous Materials Journalthe researchers used an in vitro artificial colon model plus intestinal cell cultures to examine how PM exposure influences the gut microbiota and gut barrier.[xii] “Clinical studies are limited by obvious ethical constraints. In vitro human colon models are valuable tools for maintaining a complex and metabolically active human gut microbiota for several weeks under physiologically relevant conditions. ) and co-principal investigator of this study.
Mercier-Bonin and his team used a model of the adult human colon – a bioreactor system that mimics human colonic microbiota niches – to study how repeated exposure to polyethylene microspheres affects microbial communities and their metabolic activity. Fecal microbiota samples from healthy adult donors were fermented in this model system and exposed daily to PM for two weeks. Mercier-Bonin and his team examined how the diversity and activity of these microbial communities changed. They also added microbiota samples from the bioreactor to cell cultures of donor intestinal cells obtained from cell banks to study whether exposure to PM disrupts the intestinal barrier.
The researchers found that continued exposure to PM decreased beneficial gut bacteria and increased pathogenic species, as well as their production of potentially harmful metabolites associated with gut dysregulation. These metabolites did not significantly disrupt cultured gut mucosa, but healthy adult microbiota samples used in this study may differ from individuals with underlying gastrointestinal disorders or those more vulnerable to exposure. to PMs, such as infants.[xiii],[xiv]. For example, other researchers have found a correlation between the severity of inflammatory bowel disease and fecal PM concentrations.[xv]
According to Ian Carroll, molecular microbiologist and assistant professor in the Department of Nutrition at the University of North Carolina, the gut microbiota is central to health. Any environmental or lifestyle factor that shifts the balance of gut microbiota towards pathogenic species can trigger or exacerbate disease and is therefore an important consideration. “You have to think about it in the context of the disease. Studies like this are great starting points – they are cross-sectional and will tell you what happened, but what is the consequence? Do [exposure] cause further illness? We don’t know the mechanisms and a lot more research needs to be done,” Carroll said.
As researchers continue to track the ebb and flow of gut microbial communities, marine plastic litter wrecks are sinking deeper into environmental niches and the food chain. Ultimately, the human GI jetsam may hold the key to unraveling how PM exposure shapes digestive ecosystems and overall health.
[ii] G. Erni-Cassola et al., “Distribution of plastic polymer types in the marine environment: a meta-analysis,” J Danger Mater369: 691-98, 2019.
[iii] K. Senathirajah et al., “Mass estimation of ingested microplastics – A crucial first step towards human health risk assessment”, J Danger Mater404:124004, 2021.
[iv] P. Wu et al., “Absorption, distribution, metabolism, excretion and toxicity of microplastics in the human body and health implications”, J Hazar Mater437:129361, 2022.
[v] HA Leslie et al., “Discovery and Quantification of Plastic Particle Pollution in Human Blood”, About Int163:107199, 2022.
[vi] LC Jenner et al., “Detection of Microplastics in Human Lung Tissue Using μFTIR Spectroscopy,” Sci Total Environ831:154907, 2022.
[vii] T. Horvatits et al., “Microplastics Detected in Cirrhotic Liver Tissue”, EBioMedicine82:104147, 2022.
[viii] A. Ragusa et al., “Plasticenta: first evidence of microplastics in the human placenta”, About Int146:106274, 2021.
[ix] S. Liu et al., “The association between microplastics and microbiota in placentas and meconium: the first evidence in humans”, Environ Sci Technologydoi:10.1021/acs.est.2c04706, Epub ahead of print, 2022.
[xi] P. Schwabl et al., “Detection of various microplastics in human stool: a prospective case series”, Ann Medical Intern171(7):453-57, 2019.
[xii] E. Fournier et al., “Microplastics: what happens in the human digestive tract? First evidence in adults using in vitro intestinal models”, J Danger Mater442:130010, 2023.
[xiv] S. Liu et al., “Detection of various microplastics in placentas, meconium, infant feces, breast milk and infant formula: a pilot prospective study”, Sci Total Environ854:158699, 2022.
[xv] Z. Yan et al., “Analysis of Microplastics in Human Feces Reveals Correlation Between Fecal Microplastics and Inflammatory Bowel Disease Status,” Environ Sci Technology56(1):414-21, 2022.