Impaired Efflux of the Siderophore Enterobactin Induces Envelope Stress in Escherichia coli

The Cpx response is one of several envelope stress responses that monitor and maintain the integrity of the gram-negative bacterial envelope. While several conditions that are known or predicted to generate misfolded inner membrane proteins activate the Cpx response, the molecular nature of the Cpx inducing cue is not yet known. Studies have demonstrated that mutation of multidrug efflux pumps activates the Cpx response in many gram-negative bacteria. In , pathway activation is due to accumulation of the catechol siderophore vibriobactin. However, the mechanism by which the Cpx response is activated by mutation of efflux pumps in remains unknown. Here we show that inhibition of efflux by deletion of , the outer membrane channel of several multidrug efflux pumps, activates the Cpx response in as a result of impaired efflux of the siderophore enterobactin. Enterobactin accumulation in the mutant reduces activity of the nicotinamide adenine dinucleotide (NADH) oxidation arm of the aerobic respiratory chain. However, the Cpx pathway remains active in the mutant when either NADH dehydrogenase I, NADH dehydrogenase II, or cytochrome is absent. Finally, we show that the Cpx response down-regulates transcription of the enterobactin biosynthesis operon. These results suggest that the Cpx response promotes adaptation to envelope stress in enteric bacteria that are exposed to iron-limited environments, which are rich in envelope-damaging compounds and conditions.