*3.1.3 Colorectal cancer*

Worldwide, the colorectal cancer (CRC) is the fourth most common cause of death associated with cancer [81]. Like other cancers, the CRC is a complex disease related to environmental and genetic factors. Ongoing research has proposed that gut microbiota assumes a role in the convergence of these factors, likely through forming a tumor-advancing environment.

In certain studies, by using a germ-free mice model of adenomatous polyposis coli (APC), a markedly reduced incidence of colonic tumor and a lower tumor load was revealed when compared to normally raised mice. Further other distinct CRC phenotypes such as bleeding from rectum and iron deficiency has also been shown with an invasion of inflammatory cells emerging from an intestinal epithelial barrier dysfunction. Therefore, it seems that the microbiome and host factors (for example, age and genetic predisposition) are important to the CRC growth and progression [82].
