ARTICLE ABSTRACTInflammation is a critical player in the development of both colitis-associated and sporadic colon cancers. Several studies suggest that the microbiota contribute to inflammation and tumorigenesis; however, studies to understand the role of the microbiota in colon tumor development in germ-free (GF) mice are limited. We therefore studied the effects of the microbiota on the development of inflammation and tumors in GF and conventionally raised specific pathogen-free (SPF) mice treated with azoxymethane (AOM) and dextran sulfate sodium (DSS). We discovered that GF mice developed significantly more and larger tumors compared with that in SPF mice after AOM and DSS treatment despite the lack of early acute inflammation in response to chemically induced injury by DSS. Although the extent of intestinal epithelial damage and apoptosis was not significantly different in GF and SPF mice, there was a delay in intestinal epithelial repair to DSS-induced injury in GF mice resulting in a late onset of proinflammatory and protumorigenic responses and increased epithelial proliferation and microadenoma formation. Recolonization of GF mice with commensal bacteria or administration of lipopolysaccharide reduced tumorigenesis. Thus, although commensal bacteria are capable of driving chronic inflammation and tumorigenesis, the gut microbiota also have important roles in limiting chemically induced injury and proliferative responses that lead to tumor development. Cancer Res; 73(24); 7199–210. ©2013 AACR.