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The high number of bacteria in the intestine poses a constant threat to the host. Under normal homeostatic conditions, there is a bacteria-host balance, where all the bacteria are kept at a distance from direct contact with the host cells.1 The importance of mucus has been realised for a long time, but its organisation has only recently emerged. Importantly, the small and large intestine differ in mucus organisation and how the bacterial contact with the epithelium is minimised. In the colon, the inner and attached mucus layer act as an impenetrable barrier that does not allow bacteria to enter.1 On the contrary, the outer colon mucus layer is non-adherent and penetrable to bacteria, and as such makes up the natural habitat for the commensals. The small intestine has only one mucus layer2 into which bacteria can penetrate, but the bacteria are still kept away from the epithelium by the numerous antimicrobial peptides and proteins secreted, and the forward peristaltic movement of the mucus with its trapped bacteria.3 ,4
Funding Swedish Research Council (no. 7461, 21027), The Swedish Cancer Foundation, The Knut and Alice Wallenberg Foundation, Inga Britt and Arne Lundberg Foundation, Sahlgren's University Hospital (LUA-ALF), Wilhelm and Martina Lundgren's Foundation, Söderberg's Stiftelser, The Sahlgrenska Academy, NIH (U01AI095473) and The Swedish Foundation for Strategic Research - The Mucus-Bacteria-Colitis Center (MBC) of the Innate Immunity Program.
Competing interests None.
Provenance and peer review Commissioned; externally peer reviewed.
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