Clonal Dynamics and Antimicrobial Resistance of Bloodstream Carbapenem-Resistant Acinetobacter baumannii Isolates from Korean Hospitals Between 2016 and 2020

<b>Background/Objective</b>: <i>Acinetobacter baumannii</i> is an opportunistic pathogen responsible for various healthcare-associated infections, particularly in critically ill patients. The emergence and rapid spread of multidrug- and extensively drug-resistant strains, notably carbapenem-resistant <i>A. baumannii</i> (CRAB), threaten global health. We aimed to investigate the clonal distribution, antimicrobial resistance profiles, and resistance determinants of CRAB bloodstream isolates in Korean hospitals to identify emerging high-risk clones and their potential clinical impact. <b>Methods</b>: Sequence types (STs) were determined using the Oxford multilocus sequence typing scheme, and antimicrobial susceptibility profiles and resistance determinants were evaluated. <b>Results</b>: We analyzed 812 CRAB bloodstream isolates collected from nine South Korean tertiary hospitals between 2016 and 2020. The isolates were classified into 39 STs, with ST191 (<i>n</i> = 245) and ST369 (<i>n</i> = 192) being the most prevalent. Between 2016 and 2020, ST369 increased from 2.6% to 37.9%, while ST195, first detected in 2018 (0.5%), increased to 19.0%; however, ST191 declined from 45.2% to 19.0%. Most CRAB infections were hospital-acquired (91.6%, 744 of 812), predominantly affecting men aged ≥51 years, particularly the 71-80-year-olds. Resistance rates were ≥80% for ampicillin-sulbactam and ciprofloxacin. <i>bla</i>_OXA-23 was detected in 807 isolates, confirming its central role in carbapenem resistance. ST195 exhibited higher resistance to minocycline (29.4%) than did the other STs. <b>Conclusions</b>: Dynamic clonal shifts and high antimicrobial resistance exist among CRAB isolates in Korean hospitals, with the rapid emergence of ST195 and ST369 increasing clinical challenges. Continuous epidemiological surveillance and targeted infection control measures are essential to control the spread of high-risk CRAB clones.