Figure 3.

The mechanism underlying mucin degradation in healthy individual and IBD patient. In healthy individual, some commensal bacteria can bind to the outer mucus gel layer and act as a defensive barrier to resist pathogenic bacteria. At the same time, some short-chain fatty acids get through the mucus gel layers and epithelium to provide energy for mucus degradation, which is the first barrier between the lumen and the mucosa (A). When inflammation occurs in IBD patient, some oligosaccharides derived from the degraded mucus offer energy to the mucus-degrading bacteria (like Rumminococcus gnavus and Rumminococcus torques); then, the invading bacteria change the mucus gel layer's structure, and pathogenic bacteria are now able to bind to and degrade the structure of the layers and invade the epithelium (B).