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. Unfortunately, global increase in clinical outbreaks of colistin and carbapenem resistant A. baumannii infections is on the rise and cause public health concern. In the present study, a total of 187 A. baumannii recovered from specimens of 240 patients admitted to intensive care units (ICUs) of two hospitals in Kerman, Iran during 2017-2018. Among the isolates, we found four extensive drug-resistant (XDR) with Minimum Inhibitory Concentration (MIC) >= 4 mu g/mL against colistin. The colistin-resistant (Col-R) isolates harbored blaOXA-51 and blaOXA-23 carbapenemase genes, exhibited resistance to all antibiotic classes except tigecycline and ampicillin-sulbactam. They belonged to clonal complex 2, a new MLST type 1752 and displayed identical RAPD-PCR fingerprints. Phylogenetic tree analysis suggested that, the Col-R A. baumannii emerged by endogenous mutations rather than acquisition of preexisting clones. Expressions of pmrCAB by quantitative real-time PCR (qRT-PCR) revealed 8 and 7 folds increased in the transcription levels of pmrB/C genes in strain 1 grown in presence of 16 mu g/mL colistin (p <= 0.01). However, no change in the expression of the pmrA was observed. Furthermore, DNA sequencing of Col-R genes illustrated three nonsynonymous substitutions in the LpxA (N136 -> K), LpxC (P293 -> Q), and PmrB (V21 -> F, S28 -> R, I149 -> F) in the strains 1 and 3, respectively showing MIC 32 mu g/mL against colistin. Multiple amino acids alignments demonstrated several substitutions in N-terminal region of PmrB. In conclusion, the above results provide valuable insights into the mechanism of Col-R in A. baumannii and the expressions of relative genes.