Geographic Stratification and Pangenome Plasticity Drive the Global Success of Carbapenem-Resistant Acinetobacter Baumannii ST2

The global success of carbapenem-resistant Acinetobacter baumannii sequence type 2 (ST2) has been linked to extensive genomic plasticity, yet the mechanisms stabilizing resistance remain poorly defined. This study integrated bronchoalveolar lavage fluid metagenomics, whole-genome sequencing (WGS), and global phylogenomics to resolve the evolutionary architecture of carbapenem-resistant Acinetobacter baumannii. Local ST2 populations were polyclonal and uniformly exhibited chromosomal integration of the gene blaOXA-23 via the Tn2006 composite transposon. Metagenomic analyses revealed dense in situ cooccurrence networks comprising 34 resistance genes. At the global scale, Chinese isolates carried significantly higher resistance gene burdens than international counterparts. Structural analyses identified the replication-termination T1 and T2 plasmids as horizontal “shuttles” facilitating accessory gene dissemination, whereas chromosomal insertion sequences functioned as vertical “anchors” stabilizing core resistance loci. These findings supported a shuttle-and-anchor evolutionary model underlying ST2 persistence and regional endemic adaptation.