The (un)successful global spread of Acinetobacter baumannii ST3.

Acinetobacter baumannii sequence type 3 (ST3) has been reported in multiple regions but has not achieved the global dominance of major clones such as ST1 and ST2. Here, we analysed 383 ST3 genomes, including 205 isolates from the Multidrug-Resistant Organism Repository and Surveillance Network (MRSN) and 178 publicly available genomes, to investigate the population structure, resistance gene repertoire and evolutionary dynamics of this lineage. Phylogenetic analysis revealed two major clades with multiple subclades, with 82% of genomes linked to the Middle East, highlighting this region as the likely origin or reservoir. Despite detection across 12 countries, ST3 remains relatively uncommon globally, except in Israel where it is locally abundant. Pan-genome analysis revealed a core genome comprising ~42% of total gene content, comparable to other major sequence types. Extensive recombination was observed, including large chromosomal segments, a total of ~350 kb and ~1.2 Mb shared with ST1 and ST2, respectively, as well as recurrent exchanges at the ISAba1-ampC locus. A chromosomal resistance island, designated AbST3GRI, was identified in nearly all genomes and exhibited structural diversity driven by IS26. A total of 132 genomes carried carbapenem resistance genes, predominantly bla OXA-23. However, ST3 genomes contain a relatively limited repertoire of resistance genes compared to globally dominant clones. Despite retaining most key virulence determinants, ST3 has not undergone widespread global expansion. Our findings suggest that, although genetically dynamic and capable of acquiring resistance, the lack of extensive antibiotic resistance gene repertoire may have limited the global success of this lineage.