Background <p>Breast cancer risk and mortality are associated with disrupted gut microbiome functions which in turn can affect tumor immune responses. One source of disruption could be stress. Social isolation (SI) stress consistently increases breast cancer risk and mortality in preclinical models and women, but whether SI promotes mammary tumor growth by affecting gut microbiome has not been studied.</p> Methods <p>We investigated if increased E0771 mammary tumorigenesis in SI female C57BL/6 mice was associated with changes in their gut microbiome by treating mice with an antibiotic mix that suppresses bacterial abundance and by performing fecal microbiota transplantation (FMT) from SI or group-housed (GH) donors to GH host. The effect of SI on anti-tumor CD8 + T and immunosuppressive Foxp3 + Treg cells was also studied.</p> Results <p>Fecal bacteria that were present at different abundances between GH and SI mice were short chain fatty acid (SCFA) producers, and the most consistent change across three replicate studies was decreased fecal abundance of <i>Akkermansia</i> genus in SI mice. In addition, fecal propionic acid levels were reduced in SI mice, compared with GH mice, in agreement with <i>Akkermansia</i> being propionic acid producer. SI reduced the activation of CD8 + T cells systemically and in the tumor microenvironment, while the levels and activation of immunosuppressive Foxp3 Tregs were increased. Antibiotic treatment reversed increased mammary tumorigenesis and immunosuppression in SI mice but did not affect GH mice. Further, FMT from SI donors increased tumor growth in GH host, compared with FMT from GH donor.</p> Conclusion <p>Gut dysbiosis caused by SI may be driving their increased mammary tumorigenesis, potentially through gut dysbiosis induced immunosuppression.</p>