CpxR regulates the colistin susceptibility of Salmonella Typhimurium by a multitarget mechanism.

Background: The two-component signalling systems PmrAB and PhoPQ of Salmonella have been extensively studied with regard to colistin resistance. We previously showed that overexpressed CpxR could significantly increase the colistin susceptibility (16-fold compared with the WT strain) of Salmonella enterica serovar Typhimurium (Salmonella Typhimurium) through PmrAB and PhoPQ. Objectives: To identify the potential target genes of CpxR in PmrAB- and PhoPQ-related signalling pathways. Methods: His(6)-CpxR was prokaryotically expressed and purified by Ni-NTA resin affinity chromatography. beta-Galactosidase activity assays were conducted to investigate whether CpxR could regulate the promoters of colistin resistance-related genes (CRRGs). Electrophoretic mobility shift assays (EMSAs) were used to further detect His(6)-CpxR complexes with promoters of CRRGs. Results: We demonstrated for the first time (to the best of our knowledge) that CpxR and the AcrAB-TolC efflux pump have reciprocal effects on CRRG transcription. Additionally, CpxR could regulate the colistin susceptibility of Salmonella Typhimurium by binding directly to the promoters of phoPQ, pmrC, pmrH and pmrD at the CpxR box-like sequences or indirectly through other regulators including pmrAB and mgrB. Conclusions: CpxR could regulate the colistin susceptibility of Salmonella Typhimurium by a multitarget mechanism.