Malin Johansson et.al. recently published exciting findings regarding the relation between colonic bacteria and the different layers of secreted mucins.

The group has now shown that the colon is protected by two mucus layers, an inner attached and dense mucus layer and an outer formed from the inner layer that is free to move and has expanded in volume. Both these layer contain identical proteins and is maintained by the Muc2 mucin. The inner mucus layer is devoid of bacteria that have their habitat in the outer layer. In the absence of the Muc2 mucin, the bacteria reaches the epithelial cells, penetrate the crypts and invade epithelial cells. This causes inflammation and later on it can cause colon cancer. This is similar to the disease Ulcerative colitis that is now suggested to be caused by defects in the colon Muc2 mucus layers.

Fig: Immunostaining of colonic mucins (green) and FISH-staining of bacteria (red). The inner, firmly attached mucin layer is indicated by "s" (stratified).

Read more here.

Reference:
Johansson ME, Phillipson M, Petersson J, Velcich A, Holm L, Hansson GC. The inner of the two Muc2 mucin-dependent mucus layers in colon is devoid of bacteria. Proc Natl Acad Sci U S A. 2008:105(39):15064-9.