Impact of the intestinal microbiota in colorectal cancer

Over the last decade, the microbiota came of age and now earns its credentials in the arena of immuno-oncology to predict resistance to immunotherapy and influence treatment outcome. Given its localization, colorectal cancers (CRC) are intimately associated with the intestinal microbiome. Several studies described the deviations of the taxonomic composition of the microbiota and its metabolite products during the transition of intestinal polyposis to overt carcinogenesis. Despite the epidemiological link between biofilm formation and CRC, a clear cause-effect relationship between colonic residence of bacteria expressing genotoxins and genometabolites and CRC has not been formally proven. However, their association is very robust and argues for a critical role of genomic instability, sustained signaling in major oncogenic pathways, inflammation, and avoidance of immune destruction in the capacity of distinct bacteria to promote tumor aggressiveness and metastasis formation. Conversely, other taxa can be immunogenic and elicit the activation of follicular T helper cells and B cell responses, instrumental to mount an effective anticancer immune response that will pave the way to the success of immune checkpoint blockade. In this review, we will discuss the host-microbe interactions in CRC carcinogenesis.