Multicenter outbreak of plasmid-encoded blaOXA-72 Acinetobacter baumannii in southern Poland.

Carbapenem-resistant Acinetobacter baumannii (CRAb) causes hard-to-control healthcare outbreaks, but the mobile resistance determinants that sustain regional spread are often underrecognized. We characterized 70 CRAb isolates from 11 hospitals in southern Poland (2019-2022) using whole-genome sequencing and core genome multilocus sequence typing (cgMLST), with reconstruction of carbapenemase context and plasmid structure. All isolates belonged to international clone 2 (IC2; ST2, Pasteur scheme) or to ST425 (n = 47), ST195 (n = 19), ST208 (n = 2), and ST348 (n = 1) according to the Oxford scheme. In silico KL/OCL typing further differentiated the collection. All isolates were OCL18, whereas the K-locus distribution was dominated by KL125 (n = 48) and KL3 (n = 19), with two KL2 isolates and one KL9 isolate. blaOXA-72 was detected in 50/70 isolates (71.4%), indicating an atypically high regional dominance. The OXA-72-encoding gene was carried on the same ~10.9 kb GR2/repAci1 plasmid (p398AB) in all isolates, which was sequence-identical across centers and contained two oppositely oriented copies of blaOXA-72. Transformation of p398AB into susceptible A. baumannii ATCC17978 conferred a carbapenem-resistant phenotype. cgMLST identified a large multicenter cluster of blaOXA-72-positive isolates spanning multiple cities and hospitals, supporting inter-facility dissemination of a successful IC2 lineage, together with a conserved resistance plasmid. The strict conservation of p398AB during regional spread provides a practical target for rapid molecular screening in outbreak investigations. In contrast, blaOXA-23 and a novel blaOXA-23-like variant (blaOXA-1223) were confined to a minority of isolates. Overall, these data highlight an unusual dominance of plasmid-encoded blaOXA-72 in a multicenter IC2 outbreak and underscore the need for surveillance integrating clonal relatedness with tracking of transferable resistance plasmids.