The emergence and spread of antimicrobial resistance is a significant global challenge. To solve this emerging problem, the discovery of novel bacterial cellular targets and the critical pathways associated with antimicrobial resistance is urgently needed. In this study, we investigated the role of the two most critical extracytoplasmic stress response regulators, RpoE and CpxR, on the physiology and susceptibility of growing Salmonella enterica serovar Enteritidis cells using the polycationic antimicrobial agent, chlorhexidine (CHX). We showed that deletion of the cpxR gene significantly increased the susceptibility of this organism, whereas deletion of the rpoE gene had no effect on the pathogen’s susceptibility to the drug. It has been shown that a lack of the CpxR regulator induces multifaceted stress responses not only in the envelope but also in the cytosol, further affecting the key biomolecules, including DNA, RNA, and proteins. We showed that alterations in cellular trafficking and most of the stress responses are associated with a dysfunctional CpxR regulator during non-stressful (exponential growth phase) conditions, indicating that these physiological changes are intrinsically associated with the lack of the CpxR regulator. In contrast, induction of type II toxin-antitoxin systems and decrease of abundances of enzymes and proteins associated with the recycling of muropeptides and resistance to polymixin and cationic antimicrobial peptides were specific responses of the ∆cpxR mutant to the CHX treatment. Overall, our study provides insight into the effects of CpxR on the physiology of S. Enteritidis cells during the exponential growth phase and drug treatment, which may point to potential cellular targets for the development of an effective antimicrobial agent.          

Data from the cell growth, determination of MIC of CHX, drug treatment assays, complementation study and qRT-PCR represent the average of three biological replications. The iTRAQ data related to CHR treatments of the wild-type and cpxR mutant represent also the averages of three biological replicates, while iTRAQ data relevant to the no-CHX treatment represent the averages of two biological replicates.

The kinetic data for growth rates and drug treatment assay were analysed with a two-way analysis of variance (ANOVA), with strain and times as factors, followed by Tukey’s post hoc test at the 5% level. The residuals were checked to see that the assumptions behind the ANOVA model were satisfied. The analyses were carried out using Genstat 18^th edition (VSN International, Hemel Hempstead, UK). A value of P < 0.05 was considered statistically significant. Gene expression data were analysed by t-test. The P values for the differences in protein abundance of different samples (wild-type vs cpxR mutant and CHX treatment vs no CHX treatment) were determined using a two-tailed Student’s t-test.

These datasets represent the raw data for I) growth curves of the wild-type, rpoE and cpxR mutants, II) drug treatment assays, III) complementary assay, IV) proteomic iTRAQ data (no drug treatment), V) proteomic iTRAQ data (drug treatment) and VI) validation of proteomic data. All data are presented in Excel spreadsheet files. Based of these data, we created five figures and Tables 2 and 3.