Microplastics: fate in the human digestive environment in vitro and study of the epithelium / microbiota / mucus dialogue

Over the last half-century, plastic production has exploded resulting in environmental pollution. Accumulated plastics can be degraded into smaller pieces known as microplastics (MPs) found everywhere, and especially in the food chain. MPs have been detected in human feces and colon biopsies attesting of their transit through the gastro-intestinal tract (GIT). However, to date, little is known about the fate and potential effects of MPs in the human digestive sphere, particularly in the infant at-risk population. During their GIT travel, MPs can interact with physico-chemical digestive parameters, as well as gut microbiota, intestinal epithelium and its covering mucus layer. This joint PhD between MEDIS (Clermont-Ferrand) and Toxalim (Toulouse) aimed to investigate the fate of virgin MPs of polyethylene (PE), the most manufactured plastic polymer worldwide, in the adult and infant gut. This was studied using an original in vitro approach combining a human colon model and intestinal cells in culture. This doctoral research started with evaluating the impact of various stool conservative methods (48-h freezing -80°C, 48-h freezing -80°C with glycerol or lyophilization with maltodextrin/trehalose) on gut microbiota composition and activity in the Mucosal Artificial Colon (M-ARCOL), set-up under adult conditions. We showed that inoculating with raw frozen stool (48-h at -80°C) was the best option among those tested compared to fresh stools. Next, the M-ARCOL model was adapted to the specific toddler conditions in a new configuration termed Tm-ARCOL. This adaptation allowed to reproduce in vitro the main particularities associated to infant microbiota, such as lower bacterial diversity and higher abundance of Bifidobacteriaceae, together with higher concentrations of acetate and lower amounts of propionate and butyrate, as compared to adults. Then, we assessed the effects of a chronic ingestion of PE MPs on adult and infant microbiota using M-ARCOL and Tm-ARCOL, respectively, as well as the indirect impact of gut microbe metabolites after PE MP exposure on the intestinal barrier simulated by a co-culture of Caco-2 and mucus-secreting HT29-MTX cells. Interestingly, the effect of PE MP exposure on the gut microbiota was donor-dependent and resulted in an increase in potential pathobionts, like Enterobacteriaceae, regardless of age conditions. Regarding the activity of the gut microbiota, we showed for the first time that PE MP exposure was associated with a significant decrease in butyrate production in infant conditions, while skatole production increased significantly in adults. In contrast, no significant impact of PE MPs on the intestinal barrier, as mediated by changes in gut microbial metabolites, was evidenced. This PhD work provided pioneering and significant insights into the interactions of PE MPs with the adult and infant human gut microbiota and the intestinal barrier, filling some of the gaps in knowledge on MP behavior in the human GIT. This work contributes to a better understanding of MP health risk assessment for public policies. In the near future, our in vitro approach could be used for further investigations on more representative MP types (forms, sizes, aged and/or contaminated particles), in healthy but also disturbed situations including an altered gut barrier and dysbiotic microbiota (e.g. obesity, inflammatory bowel syndrome or inflammatory bowel diseases).