Resistance to sulbactam/durlobactam in carbapenem-resistant Acinetobacter baumannii in treatment-naive setting: in vitro data and genomic insights from Italian bloodstream isolates

Objectives: This study evaluated the in vitro activity of sulbactam/durlobactam and comparators against carbapenem-resistant Acinetobacter baumannii (CRAB) bloodstream isolates from Italy and investigated genomic mechanisms underlying resistance. Methods: A total of 110 consecutive CRAB isolates (2021-2023) were tested for susceptibility to sulbactam/durlobactam, cefiderocol, colistin, and comparators. Whole-genome sequencing (WGS) was performed on sulbactam/durlobactam-resistant isolates to characterize β-lactamase genes, outbreak dynamics, and mutations in penicillin-binding proteins (PBPs). Results: Sulbactam/durlobactam inhibited 88.7% of isolates (MIC₅₀ = 2 mg/L; MIC₉₀ > 64 mg/L), while 12.7% were resistant. Cefiderocol and colistin showed 91.8% and 96.4% susceptibility rates, respectively. All resistant isolates carried blaOXA-23, and three also harbored blaNDM-1. WGS revealed close clonal relationships (average nucleotide identity > 99%) among blaNDM-1 positive isolates (ST231) and among OXA-23-only isolates (ST837/ST369), indicating local outbreak dynamics. Comparative PBP analysis identified recurrent substitutions P112S and G137R (PBP1b), N392T and A515V (PBP3), and S329N (PBP5). Structural correlation suggests these mutations reduce β-lactam binding and may contribute to resistance, particularly when combined with NDM-1. Conclusions: This study provides early evidence of sulbactam/durlobactam resistance in treatment-naïve CRAB from Italy, driven by NDM-1 co-production and PBP alterations. The coexistence of epidemic ST231 and ST837/ST369 clones highlights the need for continuous genomic surveillance and prudent antibiotic stewardship to prevent dissemination of resistant A. baumannii lineages.