CpxR overexpression increases the susceptibility of acrB and cpxR double-deleted Salmonella enterica serovar Typhimurium to colistin

Background: Colistin has been used as the last therapeutic resort for treatment of MDR Gram-negative bacteria infections in humans. The two-component system CpxAR has been reported to contribute to the MDR of bacteria. There may be a more complex network mediated by CpxAR contributing to colistin susceptibility than previously understood. Methods: A series of AcrB or CpxR deletion mutants of a multidrug-susceptible standard strain of Salmonella enterica serovar Typhimurium (Salmonella Typhimurium) was constructed in our previous study. MICs of colistin were determined by the 2-fold serial broth microdilution method. Time-kill and survival assays were carried out with various concentrations of colistin. Growth curves and starvation survival were measured by OD600 or cfu count in LB and M9-glucose (0.2%) minimum media. Quantitative RT-PCR was used to determine the mRNA expression levels of target genes. Results: The results showed that the MIC of colistin for the CpxR-overexpressed strain JS Delta acrB Delta cpxR::kan/pcpxR was dramatically decreased (0.05 mg/L) by 16-fold compared with JS (0.8 mg/L) and JS Delta acrB Delta cpxR::kan (0.8 mg/L). Colistin time-kill and survival assays showed that JS Delta acrB Delta cpxR::kan/pcpxR was more susceptible to colistin (0.05 mg/L), but had a considerably higher survivability regarding prolonged starvation stress compared with JSDacrBDcpxR:: kan. Furthermore, the expression levels of colistin resistance-related genes (phoP, phoQ, pmrB, pmrC, pmrH and pmrD) were found to be remarkably down-regulated and the negative regulatory protein mgrB was significantly up-regulated. Conclusions: This study demonstrated that CpxR may regulate the colistin susceptibility of Salmonella Typhimurium through the PmrAB and PhoPQ regulatory systems.