Engineered E. coli Nissle 1917 has been found to deplete branched-chain amino acids, which may help suppress the development of colorectal tumors. This discovery suggests a potential mechanism by which the engineered bacteria could inhibit colorectal tumorigenesis. The depletion of branched-chain amino acids by E. coli Nissle 1917 appears to play a role in its tumor-suppressing effects.