Fig. 6.

The working model for the host factor in regulating protective microbiota, mucus layer, and epithelial barriers. In the healthy intestine, the balanced microbiota, strong mucus layer and protective epithelial barriers keep the intestine from inflammation and tumorigenesis. However, in the susceptible host (e.g., VDR deficiency, APC mutation), there is preexisting barrier dysfunction and dysbiosis. The tumor development worsens the microbial dysbiosis, narrows mucus layer, and damages barrier functions by modifying tight junctions (TJs). Moreover, these dysfunctional changes could further induce the dislocation of microbiota from the lumen to the intestinal epithelium and tumors.