Enterococcus, a genus that includes common commensal bacteria found in the gut, harbors a bacteriophage that influences the effects of various cancer immunotherapies in ways that may be clinically relevant, researchers working in mice report. The findings reveal that intestinal, microbe-specific T cell responses to bacteriophages may contribute to anticancer immune responses by cross reacting with tumor-associated antigens. This highlights microbes’ therapeutic potential in the cancer space. Several recent studies have indicated that the gut microbiota plays a role in influencing the cancer-immune set point, which describes the balance between factors that promote or suppress anticancer immunity. Thus, gut microbes have been suspected to be important in the clinical outcome of widely used cancer treatments, including chemotherapy and PD-1 blockade immunotherapy. While it’s speculated that intestinal microbes induce memory T cells that cross-react with tumor-associated antigens, the mechanisms through which microbe-specific lymphocytes contribute to antitumor immune responses remain unknown. Aurélie Fluckiger and colleagues discovered that a bacteriophage that preys upon enterococci intestinal bacteria stimulates an immune response that appears to improve the systemic immune response to anticancer treatments. Fluckiger et al. found that administration of enterococci containing the bacteriophage boosted T cell responses in mice, after cancer treatments. The authors further note that the presence of the bacteriophage in human cancer patients was associated with improved survival following PD-1 immunotherapy.