Molecular characterization of lpxACD and pmrA/B two-component regulatory system in the colistin resistance Acinetobacter baumannii clinical isolates

Colistin is drug of choice for treatment of carbapenem resistant Acinetobacter baumannii infections, but increasing colistin resistance (Col-R) has been emerged across the globe. In this study, we collected 187 A. baumannii isolates from specimens of 240 patients admitted to intensive care units (ICUs) of two hospitals in Kerman, Iran during 2017-2018. Among the isolates, four isogenic extensive drug-resistant (XDR) strains with Minimum Inhibitory Concentration (MIC) ≥4 µg/mL against colistin were selected for further study. All the Col-R isolates harbored an intrinsic blaOXA–51 and blaOXA-23 carbapenemase genes. They were resistant to all antibiotic classes except tigecycline and ampicillin-sulbactam. The Col-R isolates were belonged to clonal complex 2, a new ST type 1752 and had identical high-quality RAPD-PCR fingerprints. Phylogenetic tree analysis of PmrA/B suggested that, the Col-R A. baumannii were emerged by endogenous mutations rather than acquisition of preexisting clone. In addition, DNA sequencing of the Col-R genes, showed three different nonsynonymous substitutions in LpxA (N136→K), LpxC (P293→Q) and PmrB transmembrane motif (V21→F and S28→R) of the strains 1 and 3. Interestingly, these strains showed high level MIC against colistin (MIC 32µg/mL). Analysis of gene expression by relative quantitative real-time PCR (qRT-PCR) revealed 8- and 7.1-folds increases in the transcription levels of pmrB and pmrC genes in the strain 1 when cells grown in the presence of 16 µg/mL colistin (p≤0.01). In conclusion, the above results provide valuable insights into the mechanism of Col-R in A. baumannii and the expressions of relative genes