Bacteria are associated with certain cancers and may induce genetic instability and cancer progression. The gut microbiome modulates the response to cancer therapy. Training machine learning models with response associated taxa or bacterial genes predict patients' response to immunotherapies with moderate accuracy. Clinical trials targeting the gut microbiome to improve immunotherapy efficacy have been conducted. While single bacterial strains or small consortia have not be reported yet to be successful, encouraging results have been reported in small single arm and randomized studies using transplant of fecal microbiome from cancer patients who successfully responded to therapy or from healthy volunteers.