Background: Pancreatic cancer possesses a profoundly immunosuppressive tumor microenvironment and exhibits almost no response to immunotherapy. Therapies that might enhance anti-tumor immune responses to pancreatic cancer have the potential to significantly improve outcomes. The intestinal microbiome is known to regulate the responses of solid tumors to immunotherapy. However, it is now known where specific commensal microbiota could be harnessed as an anti-cancer-therapy. MRx0518 is a strain of the commensal bacteria, Enterococcus gallinarum, and is associated with enhanced immune function and microbiome diversity. We hypothesized that treatment with neoadjuvant MRx0518 just prior to resection could enhance immune responses to pancreatic cancer. Methods: This is a single arm, Phase I study to evaluate the safety of MRx518 with preoperative hypofractionated radiation for resectable pancreatic cancer. A total of 14 patients with resectable pancreatic cancer who received standard of care neoadjuvant chemotherapy (gemcitabine/nab-paclitaxel or mFOLFIRINOX) for a minimum of 3 months were enrolled. Patients received preoperative radiation therapy (RT) to the tumor and regional lymph nodes to a total dose of 30Gy in 10 fractions with concurrent capecitabine. All patients received oral MRx0518 twice daily one week prior to radiation, during the two weeks of RT, and then daily until the day prior to resection, which occurred 4-6 weeks after the completion of RT. Blood and stool samples will be obtained at baseline, just before treatment, but after 2 weeks of MRx518, just after SBRT, and just prior to resection. Results: A total of 14 patients were enrolled. We observed no grade 3 toxicity in any patient during the study period. Of the 14 patients placed on the study, 10 patients went on to have an R0 resection (71.4%), while 4 patients exhibited progression prior to surgery which precluded a resection. Conclusions: Together these data suggest that the addition of MRx0518 to preoperative radiation was a safe intervention. Further investigations will be needed to determine how MRx0518 modulated the immune responses in the resected tumor. Clinical trial information: NCT04193904.