The Lon AAA+ protease degrades cellular abnormal proteins, such as those damaged and/or mis-synthesized, in its intramolecular chamber. The Lon activity must be precisely controlled, but the mechanism by which Lon is regulated in response to different environments is not known. Enterobacteriaceae members, including E. coli, Klebsiella, Salmonella, Serratia, Shigella and Yersinia pestis, are the most pathogenic and the most-frequently encountered organisms in clinical microbiology. They are facultative anaerobes, and encounter both anaerobic and aerobic environments inside and outside the host′s body, respectively. Characteristically, the bacteria have two cysteine residues on the Lon protease (P) domain surface that unusually form a disulfide bond. Here we show that the cysteine residues act as a redox switch of Lon. Upon disulfide bond reduction, the exit pore of the P-domain ring narrows by ~30%, thus interrupting product passage and decreasing activity by 80%; disulfide bonding by oxidation restores the pore size and activity. The redox switch (E°′ = −227 mV) is appropriately tuned to respond to variation between anaerobic and aerobic conditions, and thereby optimizes the cellular proteolysis level for each environment. Using this unprecedented mechanism, Enterobacteriaceae members live robustly both inside and outside the host′s body, thus frequently causing diseases.