Carbapenem-Resistant Acinetobacter baumannii in US hospitals: diversification of circulating lineages and antimicrobial resistance

Carbapenem-resistant Acinetobacter baumannii (CRAb) are a major cause of healthcare-associated infections. CRAb are typically multidrug-resistant and infection is difficult to treat. Despite the urgent threat that CRAb pose, few systematic studies of CRAb clinical and molecular epidemiology have been conducted. The Study Network of Acinetobacter as a Carbapenem-Resistant Pathogen (SNAP) is designed to investigate the clinical characteristics and contemporary population structure of CRAb circulating in US hospital systems using whole genome sequencing (WGS). Analysis of the initial 120 SNAP patients from four US centers revealed that CRAb remain a significant threat to hospitalized patients, affecting the most vulnerable patients and resulting in 24% all-cause 30-day mortality. The majority of currently circulating isolates belonged to ST2Pas, a part of Clonal Complex 2 (CC2), which is the dominant drug-resistant lineage in the United States and Europe. We identified three distinct sub-lineages within CC2, which differed in their antibiotic resistance phenotypes and geographic distribution. Most concerning, colistin resistance (38%) and cefiderocol (10%) resistance were common within CC2 sub-lineage C (CC2C), where the majority of isolates belonged to ST2Pas/ST281Ox. Additionally, we identified a newly emergent lineage, ST499Pas that was the most common non-CC2 lineage in our study and had a more favorable drug susceptibility profile compared to CC2. Our findings suggest a shift within the CRAb population in the US during the past 10 years, and emphasize the importance of real-time surveillance and molecular epidemiology in studying CRAb dissemination and clinical impact. ImportanceCarbapenem-resistant Acinetobacter baumannii (CRAb) constitute a major threat to public health. To elucidate the molecular and clinical epidemiology of CRAb in the US, clinical CRAb isolates were collected along with data on patient characteristics and outcomes and bacterial isolates underwent whole genome sequencing and antibiotic susceptibility phenotyping. Key findings included emergence of new sub-lineages within the globally predominant clonal complex (CC) 2, increased colistin and cefiderocol resistance within one of the CC2 sub-lineages, and the emergence of ST499Pas as a previously unrecognized CRAb lineage in US hospitals.