Characterization of XDR-Acinetobacter baumannii clinical isolates: prominent role of adeB and blaOXA-51 in carbapenem resistance

Introduction: Extensively drug-resistant (XDR) Acinetobacter baumannii is a global healthcare threat. This study characterized clinical XDR A. baumannii isolates from a Tehran hospital, focusing on carbapenem resistance mechanisms, virulence genes, and correlation with meropenem MICs.
Methods: Forty XDR A. baumannii isolates underwent phenotypic testing (mCIM, eCIM, DDST, meropenem MICs). PCR detected carbapenemase genes (blaOXA-51, blaNDM, blaVIM, blaIMP), ISAba1 upstream of blaOXA-51, and virulence genes (adeB, carO). qRT-PCR compared adeB and carO expression in high- vs. low-MIC isolates.
Results: All isolates were metallo-β-lactamase negative. blaOXA-51 was present in 95% (38/40), blaNDM in 20% (8/40); blaVIM, blaIMP, and ISAba1 were absent. adeB and carO occurred in 87.5% and 77.5%, respectively. All adeB-negative isolates belonged to the low-MIC group. Mean expression of adeB and carO was higher in high-MIC isolates, but differences were not significant (p>0.05).
Conclusion: Carbapenem resistance in these XDR isolates is primarily associated with intrinsic blaOXA-51 and acquired blaNDM. The high prevalence of adeB and carO suggests their contribution to resistance, but the lack of significant correlation with MICs indicates multifactorial, complex regulatory mechanisms beyond transcriptional activity.