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The inner of the two Muc2 mucin-dependent mucus layers in colon is devoid of bacteria

Johansson, Malin E.V., Phillipson, Mia, Petersson, Joel, Velcich, Anna, Holm, Lena, Hansson, Gunnar C.
Proceedings of the National Academy of Sciences of the United States of America 2008 v.105 no.39 pp. 15064-15069
bacteria, colon, direct contact, epithelial cells, epithelium, inflammation, mice, mucins, mucus, neoplasms, protein composition, proteolysis, proteomics, symbiosis
We normally live in symbiosis with ~10¹³ bacteria present in the colon. Among the several mechanisms maintaining the bacteria/host balance, there is limited understanding of the structure, function, and properties of intestinal mucus. We now demonstrate that the mouse colonic mucus consists of two layers extending 150 μm above the epithelial cells. Proteomics revealed that both of these layers have similar protein composition, with the large gel-forming mucin Muc2 as the major structural component. The inner layer is densely packed, firmly attached to the epithelium, and devoid of bacteria. In contrast, the outer layer is movable, has an expanded volume due to proteolytic cleavages of the Muc2 mucin, and is colonized by bacteria. Muc2⁻/⁻ mice have bacteria in direct contact with the epithelial cells and far down in the crypts, explaining the inflammation and cancer development observed in these animals. These findings show that the Muc2 mucin can build a mucus barrier that separates bacteria from the colon epithelia and suggest that defects in this mucus can cause colon inflammation.