Mechanistic and clinical insights into a PmrB mutation driving colistin resistance and virulence in Acinetobacter baumannii

The global rise of colistin-resistant Acinetobacter baumannii (CRAB) is a critical health threat, particularly in the Middle East. The pmrB c.235T > A (p.Leu79Ile) mutation is frequently observed in clinical isolates, but its comprehensive characterization is lacking. In this multicenter study, 465 clinical A. baumannii isolates were collected from Iran (n = 378) and Iraq (n = 87) between January 2023 and June 2024. Colistin susceptibility was determined by broth microdilution, and the pmrCAB operon was sequenced. The structural impact of p.Leu79Ile was assessed via 200-ns molecular dynamics simulations. Functional consequences were evaluated using lipid A profiling, biofilm assays, serum resistance, and a murine pneumonia model. Phylogeographic analysis and a LASSO-regularized logistic regression model for 30-day mortality prediction were performed. Colistin resistance was detected in 47.8% of isolates, with 94.2% of resistant isolates harboring pmrCAB mutations; c.235T > A was predominant (76.8%). Simulations indicated that Leu79Ile stabilizes the active conformation of PmrB (ΔΔG = -3.8 kcal/mol), increasing the root mean square deviation (RMSD; 4.7 Å vs. 2.3 Å) and altering protein dynamics. This correlated with a 1.93-fold increase in pEtN-modified lipid A and a strong inverse correlation with colistin MIC (ρ = -0.89). Mutant isolates exhibited enhanced biofilm formation (2.3-fold), serum survival (47.6% increase), and murine mortality (70% vs. 30%). Phylogenetics identified a dominant ST848-bla ~ OXA-23~^+^ clone emerging around 2016 (95% HPD: 2015-2017), with evidence of Iran-Iraq cross-border transmission. The mortality prediction model (predictors: age > 65, CCI > 3, ventilation, shock, c.235T > A, ST848) achieved an AUC = 0.87 in validation. This study suggests the pmrB c.235T > A mutation confers a dual phenotype of colistin resistance and hypervirulence, driven by structural stabilization of PmrB within an expanding regional clone.