Gut microbiota: What impact on colorectal carcinogenesis and treatment?

The gut microbiota, composed of 10(14) microorganisms, is now considered as a "hidden organ'', regarding to its digestive, metabolic and immune functions, which are helpful to its host. For the last 15 years, advances in molecular biology have highlighted the association of gut microbiota dysbiosis with several diseases, including colorectal cancer. An increased abundance of some bacteria (including Fusobacterium nucleatum, Bacteroides fragilis, Escherichia coli) is associated with cancer, whereas others seem to be protective (Faecalibacterium prausnitzii). Several mechanisms, which are species-specific, are involved in colorectal carcinogenesis. Most of the time, bacterial toxins are involved in pro-inflammatory processes and in activation of angiogenesis and cellular proliferation pathways. The identification of these bacteria leads to envisage the gut microbiota as potential screening tool for colorectal cancer. Recent studies showed a relation between the gut microbiota and the efficacy and toxicity of chemotherapies (oxaliplatin, irinotecan) and immunotherapies (including ipilimumab). Therapeutic approaches targeting the gut microbiota are now available (probiotics, fecal microbiota transplantation...). New therapeutic strategy combining both chemotherapy and/or immunotherapy with an adjuvant treatment targeting the gut microbiota can now be developed in order to improve treatment response and tolerance.