New isolates from the 1970s to early 2000s provide insights into the evolution of Acinetobacter baumannii international clone 2 and its resistome.

Acinetobacter baumannii remains a pervasive nosocomial pathogen, with international clone 2 (IC2) being the most successful lineage. IC2 is often discussed as a singular entity, and the genomic factors behind its success over other lineages are poorly characterized. This study aimed to track genomic changes occurring around the expansion of IC2 and further determine the within-clone population structure. A set of historical A. baumannii isolates from the 1970s to early 2000s was long-read sequenced and assembled into high-quality chromosomes. These were then used to supplement publicly available A. baumannii genomes, producing a dataset of 1,281 chromosomes derived from isolates collected across 6 continents. A recombination-free phylogeny was produced and combined with antimicrobial resistance gene presence/absence data. Analysis showed that IC2 became the dominant lineage around 2005, which coincides with the acquisition of oxa23 and AbGRI3. Further, IC2 can be split into at least four distinct groups. Groups 1, 2 and 3 represent incremental evolution of the A. baumannii chromosome over time, while group 4 represents a separate evolutionary path distinct from the main IC2 lineage, which has recently been isolated at higher frequency.