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Review
. 2018 Jul;20(7):2337-2353.
doi: 10.1111/1462-2920.13926. Epub 2017 Nov 10.

Host-microbe interaction in the gastrointestinal tract

Aimée Parker  1 , Melissa A E Lawson  1 , Laura Vaux  1 , Carmen Pin  1
Affiliations
Review

Host-microbe interaction in the gastrointestinal tract

Aimée Parker et al. Environ Microbiol. 2018 Jul.

Abstract

The gastrointestinal tract is a highly complex organ in which multiple dynamic physiological processes are tightly coordinated while interacting with a dense and extremely diverse microbial population. From establishment in early life, through to host-microbe symbiosis in adulthood, the gut microbiota plays a vital role in our development and health. The effect of the microbiota on gut development and physiology is highlighted by anatomical and functional changes in germ-free mice, affecting the gut epithelium, immune system and enteric nervous system. Microbial colonisation promotes competent innate and acquired mucosal immune systems, epithelial renewal, barrier integrity, and mucosal vascularisation and innervation. Interacting or shared signalling pathways across different physiological systems of the gut could explain how all these changes are coordinated during postnatal colonisation, or after the introduction of microbiota into germ-free models. The application of cell-based in-vitro experimental systems and mathematical modelling can shed light on the molecular and signalling pathways which regulate the development and maintenance of homeostasis in the gut and beyond.

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Figures

Figure 1
Figure 1
The surface of the intestinal epithelium is covered by two layers of mucus of varying thickness throughout the gastrointestinal tract. The outer, looser layer is colonised with bacteria, some of which use mucin as a food source. This layer is thickest in the colon, which contains the highest bacterial load. The inner layer is more firmly attached to the epithelium. In the colon, this layer is effectively sterile, whereas in the small intestine it has been proposed this inner layer may be penetrable to some bacteria.
Figure 2
Figure 2
Major components of the ileal mucosa and differences between conventionally raised and germ‐free mice. The presence of microbiota influence mucus composition, crypt‐villus morphology, epithelial immune receptor expression and AMPs release, immune structure and cell composition, vascularisation, innervation, glial networks and mucosal thickness. SIgA: Secretory Immunoglobulin A, PP: Peyer's patch, AMPs: antimicrobial peptides.
Figure 3
Figure 3
A. The intestinal epithelial barrier has multiple mechanisms for detecting and limiting bacterial invasion. Under homeostasis, release of Immunoglobulin A (IgA) and antimicrobial peptides (AMPs) into the mucus layer prevents most bacteria reaching the epithelial surface. Bacterial metabolites or peptides are bound by an array of surface receptors on epithelial cells, of which just a few examples are shown for simplicity. These include G‐protein‐coupled‐receptors (GPCRs) which bind short chain fatty acids (SCFA) and TLRs which detect lipopolysaccharide (LPS), lipoproteins and flagellin. Microbial signalling to epithelial cells can be relayed to the underlying immune and nervous systems to alter gut functions. The lamina propria is surveyed by many lymphocytes, phagocytic cells and other immune effector cells (not shown) and there is low production of inflammatory cytokines; B. Bacteria have evolved multiple methods of subverting epithelial defences and translocating to the lamina propria. Disrupted tight junctions, inflammatory signalling and epithelial cell death create gaps in the barrier allowing the entry of opportunistic bacteria, antigens and toxins from the lumen, which further amplify inflammatory responses. High levels of inflammatory cytokines, reduced mucus production and impaired antimicrobial production allow additional bacteria to reach and traverse the epithelial barrier leading to intestinal and potentially systemic infections and contributing to inflammatory bowel diseases.
Figure 4
Figure 4
Examples of in vitro technologies for studying microbial interaction with epithelium and other cells of the intestine. HIOs: human intestinal organoids, hPSCs: human pluripotent stem cells
See this image and copyright information in PMC

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