Deciphering the Colorectal Cancer Gut Microbiota: Association vs. Causality

Purpose of Review Studies have identified differences between the gut microbiota of colorectal cancer (CRC) patients versus healthy individuals. In this review, we assess the scientific literature to determine if gut microbes should be considered causal, co-varying, or a necessary but not sufficient agent in CRC development. Recent Findings Oral bacteria may influence CRC susceptibility. Colonic biofilms in both sporadic and hereditary CRC suggest these bacteria are present in early neoplasia. Pathogenic drivers and opportunistic passenger bacteria may underlie direct effect of the gut microbiota on carcinogenesis. Summary Members of multiple bacterial taxa have been implicated in CRC tumorigenesis and progression, with distinct mechanisms of action described for each. Individual bacterial organisms found in the colon are likely not enough to explain CRC development and progression. The entire colonic environment, including genetic factors, local tissue inflammatory state as well as dietary components may influence the way epithelial cells respond to the presence of certain bacteria. Longitudinal, human intervention studies are needed to completely clarify complex interactions in the colonic environment and specific causative pathways between the microbiota and CRC.