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: His6-CpxR was prokaryotically expressed and purified by Ni-NTA resin affinity chromatography. ß-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 His6-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.

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 JSΔacrBΔcpxR::kan/pcpxR was dramatically decreased (0.05 mg/L) by 16-fold compared with JS (0.8 mg/L) and JSΔacrBΔcpxR::kan (0.8 mg/L). Colistin time-kill and survival assays showed that JSΔacrBΔcpxR::kan/pcpxR was more susceptible to colistin (0.05 mg/L), but had a considerably higher survivability regarding prolonged starvation stress compared with JSΔacrBΔcpxR::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.

Molecular Characterization of a Novel Integrative Conjugative Element ICEHpa1 in Haemophilus parasuis.

ICEHpa1 was identified in the genome of a serovar 8 Haemophilus parasuis ST288 isolate YHP170504 from a case of swine lower respiratory tract infection. The aim of the present study was to characterize the integrative conjugative element ICEHpa1 and its multiresistance region. Susceptibility testing was determined by broth microdilution and the complete ICEHpa1 was identified by WGS analysis. The full sequence of ICEHpa1 was analyzed with bioinformatic tools. The presence of ICEHpa1, its circular intermediate and integration site were confirmed by PCR and sequence analysis. Transfer of ICEHpa1 was confirmed by conjugation. ICEHpa1 has a size of 68,922 bp with 37.42% GC content and harbors 81 genes responsible for replication and stabilization, transfer, integration, and accessory functions, as well as seven different resistance genes [bla Rob- 3, tet(B), aphA1, strA, strB, aac(6)'-Ie-aph(2')-Ia, and sul2]. Conjugation experiments showed that ICEHpa1 could be transferred to H. parasuis V43 with frequencies of 6.1 × 10-6. This is the first time a multidrug-resistance ICE has been reported in H. parasuis. Seven different resistance genes were located on a novel integrative conjugative element ICEHpa1, which suggests that the ICEHpa1 is capable of acquiring foreign genes and serving as a carrier for various resistance genes.

Characterization of the IncHI2 plasmid pTW4 harboring tet(M) from an isolate of Escherichia coli ST162.

To investigate the genetic features and biological costs of the plasmid pTW4 harboring tet(M) in an isolate of Escherichia coli ST162 from a duck. The complete nucleotide sequence of plasmid pTW4 was determined. The characteristics of plasmid pTW4 in E. coli were investigated by stability and direct competition assays. pTW4 is an IncHI2-type plasmid that contained the resistant genes tet(M), floR, strAB, sul2, rmtB, and blaCMY-2. Tet(M) is located in the composite transposon Tn6539 within the multidrug resistant (MDR) region on this plasmid. Furthermore, the resistance gene rmtB and blaCMY-2 were found outside the MDR region. The plasmid pTW4 remained stable in the host strain E. coli J53 after passage under an antibiotic-free environment for 7 days. However, the strain E. coli J53/pTW4 showed a fitness disadvantage of 6% per ten generations in the process of growth competition with E. coli J53. In conclusion, the plasmid pTW4, a mobile MDR vehicle, may promote the dissemination of tet(M), floR, rmtB, strAB, sul2, and blaCMY-2 among bacteria and then, but it appears to confer growth disadvantage to the host.

Identification and prevalence of RND family multidrug efflux pump oqxAB genes in Enterococci isolates from swine manure in China.

PURPOSE: The resistance/nodulation/cell division (RND) family multidrug efflux pump, OqxAB, has been identified as one of the leading mechanisms of plasmid-mediated quinolone resistance and has become increasingly prevalent among Enterobacteriaceae in recent years. However, oqxAB genes have not yet been reported in Enterococcus isolates. The aim of the present study was to identify the oqxAB genes and investigate their prevalence among Enterococcus from swine manure in China. METHODOLOGY: The oqxAB genes were screened in 87 Enterococcus isolates by PCR. The transferability of the oqxAB genes in Enterococcus was determined by conjugation experiments. The genetic environment of oqxAB genes was investigated by cloning experiments, PCR mapping and sequencing. RESULTS: A high prevalence (86.2 %) of olaquindox resistance was observed in Enterococcus and 98.9 % isolates exhibited multidrug-resistance phenotypes. The occurrence of oqxA and oqxB in Enterococcus was also high (79.3 and 65.5 %, respectively). Sequence analysis of the cloned fragment indicated that the oqxAB cassette was linked to an incomplete Tn5 transposon containing aph(3')-IIa and flanked by IS26 [IS26-oqxAB-IS26-aph(3')-IIa]. The oqxAB-aph(3')-IIa-positive transconjugant or transformant showed resistance or reduced susceptibility to enrofloxacin, ciprofloxacin, olaquindox, mequindox, florfenicol, neomycin and kanamycin. CONCLUSION: This is the first time that the oqxAB genes have been identified in Enterococcus faecalis from swine manure. The genetic linkage of oqxAB-aph(3')-IIa in Enterococcus has not been described before. The high prevalence of oqxAB genes in Enterococcus suggests that it may constitute a reservoir for oqxAB genes and pose a potential threat to public health.