A child is not an adult: development of a new in vitro model of the toddler colon.

Early life is a critical period where gut ecosystem and functions are being established with significant impact on health. For regulatory, technical, and cost reasons, in vitro gut models can be used as a relevant alternative to in vivo assays. An exhaustive literature review was conducted to adapt the Mucosal Artificial Colon (M-ARCOL) to specific physicochemical (pH, transit time, and nutritional composition of ileal effluents) and microbial parameters from toddlers in the age range of 6 months-3 years, resulting in the Tm-ARCOL. In vitro fermentations were performed to validate this newly developed colonic model compared to in vivo toddler data. Results were also compared to those obtained with the classical adult configuration. Fecal samples from 5 toddlers and 4 adults were used to inoculate bioreactors, and continuous fermentations were performed for 8 days. Gut microbiota structure (lumen and mucus-associated microbiota) and functions (gas and short-chain fatty acids) were monitored. Clearly distinct microbial signatures were obtained between the two in vitro conditions, with lower α-diversity indices and higher abundances of infant-related microbial populations (e.g., Bifidobacteriaceae, Enterobacteriaceae) in toddler versus adult conditions. In accordance with in vivo data, methane was found only in adult bioreactors, while higher percentage of acetate but lower proportions of propionate and butyrate was measured in toddlers compared to adults. This new in vitro model will provide a powerful platform for gut microbiome mechanistic studies in a pediatric context, both in nutritional- (e.g., nutrients, probiotics, prebiotics) and health-related (e.g., drugs, enteric pathogens) studies. KEY POINTS: • Development of a novel in vitro colonic model recapitulating the toddler environment. • Specific toddler versus adult digestive conditions are preserved in vitro. • The new model provides a powerful platform for microbiome mechanistic studies.