The cancer chemotherapeutic 5-fluorouracil is a potent inhibitor of Fusobacterium nucleatum and its activity is modified by the intratumoral microbiota

Fusobacterium nucleatum is among the most prevalent and dominant bacterial species in colorectal cancer (CRC) tumor tissue, and growing evidence supports its role in cancer progression and poorer patient prognosis. Here we perform a small molecule inhibitor screen of 1,846 bioactive compounds against a CRC isolate of F. nucleatum and find that 15% of inhibitors are antineoplastic agents including fluoropyrimidines. Validation of these findings reveal that 5-fluorouracil (5-FU), the first-line chemotherapeutic used to treat CRC worldwide, is a potent inhibitor of F. nucleatum CRC isolates at concentrations found in serum of CRC patients treated with 5-FU. We also identify members of the intratumoral microbiota that are resistant to 5-FU, including Escherichia coli. Further, CRC E. coli isolates can modify 5-FU and relieve 5-FU toxicity towards otherwise-sensitive F. nucleatum and human CRC epithelial cells. Lastly, we demonstrate that ex-vivo CRC tumor microbiota from patients undergo different levels of community disruption after 5-FU exposure and have the potential to deplete 5-FU levels, thereby reducing local drug efficacy. Together, these observations argue for further investigation into the role that the CRC intratumoral microbiota plays in patient response to chemotherapy.