Ecological interactions between host, commensal and pathogenic bacteria in models for the intestinal epithelium.

Gut host physiology and the microbiome intricately interact in the complex ecosystem of the human digestive tract, playing a crucial role in maintaining overall health. In recent decades, the role of the gut microbiota in the defence against pathogens and modulating local and distal immunity has been well-established. The interactions between commensal and potential pathogenic bacteria with the intestinal epithelium can initiate immune responses in the epithelial cells, which, in turn, activate downstream immune responses in other immune cells. These intricate processes involved, especially when multiple microorganisms are present as seen in the intestinal microbiome, remain only partially understood. Previously, it was observed that in adults aged 60 years or older, the commensal Ruminococcus torques (Rt) and Escherichia coli were associated with influenza-like illness and a heightened pro-inflammatory immune profile. In this study, we used a CaCo-2 cell-based model and a human intestinal enteroid (HIE) model to explore epithelial responses to Rt and an adherent invasive E. coli (AIEC) both individually and in co-cultures under anaerobic conditions. Additionally, CaCo-2 cells were co-cultured with peripheral blood mononuclear cells, revealing downstream activation of immune cells. While both systems showed comparable cytokine profiles, they differed in their responses to the different bacteria, with the organoid system being more representative for intestinal epithelial cells in humans. We provide mechanistic evidence of the pro-inflammatory responses associated with these bacteria in the intestinal ecosystem. These models, particularly in the context of combined infections, represent a valuable and promising avenue for future research. They contribute to a deeper understanding of the complex interactions between the gut microbiota, epithelial intestinal cells and immune cells in the gut ecosystem, thereby promoting advances in the field of gut health and host response.