The microbiota is emerging as an important environmental factor influencing several functions of our body. Many disorders have been associated to a disequilibrium of the microbiota that is called dysbiosis.  Colorectal cancer (CRC) is the third leading cause of cancer-related deaths in industrialized countries. It is a multifactorial disorder influenced by genetic, environmental and lifestyle factors, including excessive alcohol use, smoking, dietary habits and reduced physical activity. Dysbiosis has also been associated with the pathology, but whether this is a cause or an effect of the disease is less clear. Reports in animal models have shown that several bacterial strains may promote tumor development via increasing the stability of oncogenes, releasing genotoxic toxins, driving cell proliferation and accumulation of suppressive immune cells within the tumor or via an exacerbation of the inflammatory response. However, contrasting results have been obtained on the role of the microbiota using germ-free mice. In some models the microbiota seems to be protumorigenic, while in others antitumorigenic. This suggests also the existence of tumor-protecting bacterial strains, whose identity remains to be identified. We have studied the role of the microbiota in the ApcMin/+ mouse model of spontaneous intestinal tumorigenesis and demonstrated that dysbiosis and changes in mucus composition are concomitant to tumorigenesis. We identified a taxon most strongly underrepresented during tumorigenesis. Reconstitution of ApcMin/+ mice with a C57BL/6 mice isolate of this taxon protected mice from tumor development indicating the existence of tumor-protecting bacteria. These bacteria have a direct effect on tumor cell proliferation.