ABSTRACTUp to 25% of theE. colistrains isolated from the feces of healthy humans harbor thepksgenomic island encoding the synthesis of colibactin, a genotoxic metabolite. Evidence is accumulating for an etiologic role of colibactin in colorectal cancer. Little is known about the conditions of expression of colibactin in the gut. The intestine is characterized by a unique oxygenation profile, with a steep gradient between the physiological hypoxic epithelial surface and the anaerobic lumen, which favors the dominance of obligate anaerobes. Here, we report that colibactin production is maximal under anoxic conditions and decreases with increased oxygen concentration. We show that the aerobic respiration control (ArcA) positively regulates colibactin production and genotoxicity ofpks+ E. coliin response to oxygen availability. Thus, colibactin synthesis is inhibited by oxygen, indicating that thepksbiosynthetic pathway is adapted to the anoxic intestinal lumen and to the hypoxic infected or tumor tissue.