Outpatient clonal propagation propelled rapid regional establishment of an emergent carbapenem-resistant Acinetobacter baumannii lineage ST499Pas.

Eliminating carbapenem-resistant Acinetobacter baumannii (CRAb) disease requires comprehensive knowledge of how this non-commensal organism propagates among at-risk hosts. We molecularly characterized an ongoing surge of CRAb cases among patients in a Midwest USA healthcare system, which coincided with sustained reductions in hospital-acquired CRAb infections and falloffs of cases associated with distinctly more resistant antibiotypes. Genome sequencing revealed surge isolates belonged to an emergent Pasteur scheme sequence type 499 (ST499Pas) and comprised multiple contemporaneous clonal clusters. Detailed query of health records revealed no consistent hospital source but instead identified various outpatient healthcare settings linked to cluster cases. We show CRAb can rapidly establish a regional presence even without gains in breadth of antibiotic resistance and negligible contribution from sustained intrahospital transmission. As CRAb lineages may sidestep control efforts via outpatient epidemiological niches, our approach can be implemented to investigate outpatient CRAb propagation and inform subsequent local surveillance outside of hospital settings.