Abstract
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 Vibrio cholerae, 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 Escherichia coli remains unknown. Here we show that inhibition of efflux by deletion of tolC, the outer membrane channel of several multidrug efflux pumps, activates the Cpx response in E. coli as a result of impaired efflux of the siderophore enterobactin. Enterobactin accumulation in the tolC mutant reduces activity of the nicotinamide adenine dinucleotide (NADH) oxidation arm of the aerobic respiratory chain. However, the Cpx pathway remains active in the tolC mutant when either NADH dehydrogenase I, NADH dehydrogenase II, or cytochrome bo3 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.
Highlights
In order for antimicrobial compounds to gain access to their cellular target, they must first cross one or more layers of the bacterial envelope
We show that the catechol siderophore enterobactin is required for activation of the Cpx response in E. coli lacking tolC, suggesting that envelope damage inflicted by impaired secretion of siderophores is a conserved Cpx-inducing signal
These results suggest that the metabolite(s) responsible for activating the Cpx response in the E. coli tolC mutant is produced in minimal medium, but not in rich medium
Summary
In order for antimicrobial compounds to gain access to their cellular target, they must first cross one or more layers of the bacterial envelope. Blocking metabolite secretion by mutating tolC or TolC-dependent efflux systems increases sensitivity to cysteine, the siderophore enterobactin, and intermediates of heme biosynthesis, suggesting that metabolite accumulation is toxic (Tatsumi and Wachi, 2008; Wiriyathanawudhiwong et al, 2009; Vega and Young, 2014) In support of this hypothesis, numerous cellular stress responses are activated in bacteria lacking tolC (Rosner and Martin, 2009; Guest and Raivio, 2016a), including the Cpx envelope stress response. We show that the catechol siderophore enterobactin is required for activation of the Cpx response in E. coli lacking tolC, suggesting that envelope damage inflicted by impaired secretion of siderophores is a conserved Cpx-inducing signal. We provide evidence to suggest that activation of the Cpx response facilitates adaptation to toxic envelope stresses, such as enterobactin accumulation, by down-regulating the transcription of genes involved in enterobactin biosynthesis
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