INTRODUCTION
Clinical Acinetobacter baumannii isolates are frequently resistant to a wide spectrum of antimicrobial agents, including aminoglycosides, β-lactams, β-lactam/β-lactamase inhibitors, carbapenems, cephalosporins, and fluoroquinolones. This broad resistance underlies the emergence of multidrug-resistant, extensively drug-resistant (XDR), and pandrug-resistant phenotypes (1, 2, 3, 4, 5). Over the past two decades, carbapenem-resistant A. baumannii (CRAB) has emerged as a major global nosocomial pathogen (6, 7). The World Health Organization has classified CRAB as a critical priority pathogen requiring urgent development of new therapeutics (8). Carbapenem resistance in A. baumannii results from both intrinsic mechanisms, such as OXA-51, and the acquisition of carbapenemase genes encoding Ambler class B and D β-lactamases (5, 9, 10). Class D OXA-type β-lactamases, including OXA-23-, OXA-24-, and OXA-58-family enzymes, are primarily responsible for carbapenem resistance in global epidemic A. baumannii strains (5, 10, 11). Additionally, class B metallo-β-lactamases (MBLs), such as IMP-, VIM-, and NDM-variants, have also been implicated in carbapenem resistance of CRAB strains (5, 10, 12, 13).
Molecular epidemiological typing methods for differentiating bacterial clones have progressed from band-based typing techniques, such as pulsed-fieled gel electrophoresis (PFGE) and amplified fragment length polymorphism, to sequence- based approaches, including multi-locus sequence typing (MLST) and whole genome sequencing (14, 15, 16, 17, 18). Two MLST schemes, including the Pasteur and Oxford schemes, are widely employed for epidemiological investigation of A. baumannii(17, 19). A diverse array of sequence types (STs) has been identified among clinical A. baumannii isolates (https://pubmlst.org/organisms/acinetobacter-baumannii). Molecular typing of A. baumannii isolates has identified nine distinct clonal lineages, referred to as international clones (IC) 1 to 9 (20, 21, 22). Among these, IC2 is the most prevalent clonal lineage of A. baumannii globally and is strongly linked to multidrug-resistance phenotypes (20, 23).
This review summarizes a 15-year molecular epidemiological study of CRAB isolates collected from Kyungpook National University Hospital in Daegu, Korea (24, 25, 26, 27). During this period, class D OXA-type β-lactamases—specifically the intrinsic OXA-51 and acquired OXA-23—were the exclusive carbapenemase types identified in A. baumannii isolates. Notably, no MBLs were detected. Following the first identification of blaOXA-23 in 2004, we observed dynamic clonal shifts in clonal composition of CRAB strains, driven primarily by allelic variation that led to the emergence and replacement of STs. As CRAB clones differ in antimicrobial resistance phenotypes, virulence potential, and environmental adaptability, continuous molecular surveillance is crucial for effective infection control and for guiding the selection of appropriate therapeutics for treating patients. In this review, we detail the evolutionary trends in CRAB isolates in the study hospital from 2004 to 2018, with a focus on clonal dynamics and its association with antimicrobial resistance.
EPIDEMIOLOGY AND ANTIMICROBIAL SUSCEPTIBILITY OF A. baumannii STRAINS CARRYING blaOXA-23
Emergence of blaOXA-23-Harboring A. baumannii Strains in Korea
Globally, carbapenem resistance among Acinetobacter species has been increasing, rising from 14% in 1998 to 35% in 2006 (28, 29). In Korea, CRAB was first reported in the late 1990s, with a prevalence of 6.3% in hospitals in Seoul as early as 1999 (30). Since then, the incidence of CRAB has risen steadily. According to recent surveillance data from 2023, carbapenem resistance rates now exceed 87.4% among A. baumannii isolates in general hospitals nationwide (31). The carbapenemase genes carried by CRAB strains in Korea have also evolved over time. In the 1990s, MBL genes, such as blaVIM-2 and blaIMP-1, were commonly identified in carbapenemase-producing A. baumannii isolates (32, 33, 34). These MBL genes were typically located within class 1 integrons as gene cassettes (35). blaOXA-23 is intrinsic carbapenem-hydrolyzing oxacillinase carried by the chromosomes of A. radioresistens(36). Interestingly, blaOXA-23 was first identified in an imipenem-resistant A. baumannii 6B92 isolate recovered in 1985 from a patient in Edinburgh, Scotland, prior to the clinical use of carbapenems in this country (37). In Korea, blaOXA-23 was first detected in 2003 in 36 of 52 imipenem-resistant A. baumannii isolates collected from a tertiary hospital in Busan (38). However, microbiological characteristics of these CRAB isolates, including STs and the genetic contexts of blaOXA-23, remains uncharacterized. Since 2004, CRAB strains harboring blaOXA-23 have been reported widely across Korea. Despite variations in antimicrobial prescribing practices, healthcare infrastructure, and infection control measures among hospitals, CRAB strains carrying blaOXA-23 have become endemic nationwide.
Emergence of blaOXA-23-Harboring ST353 CRAB Strains in the Study Hospital
Following the initial detection of blaOXA-23 in Korean CRAB isolates in 2003, a surveillance study was conducted to monitor the emergence of this resistance gene in CRAB isolates collected from Kyungpook National University Hospital. Among 107 A. baumannii isolates collected between 2004 and 2005, seven (6.5%) were resistant to imipenem (39). All seven isolates were positive in the modified Hodge test but were negative in the imipenem-ethylene-diamine-tetraacetic acid double disk synergy test, indicating the production of class D or non-metallo carbapenemases. Of these seven imipenem-resistant isolates, six harbored both blaOXA-23 and blaOXA-51, while one carried only blaOXA-51. These findings represent the first identification of blaOXA-23-harboring CRAB strains in the study hospital. The first blaOXA-23-harboring CRAB isolate was recovered from a patient in the neurosurgery ward in December 2004. Molecular typing classified the six CRAB isolates harboring blaOXA-23 into two PFGE patterns and assigned them to specific STs. According to the Pasteur MLST scheme, all six isolates belonged to ST2 (2-2-2-2-2-2-2; fusA-gltA-pyrG-recA-rplB-rpoB). Under the Oxford scheme, these isolates were assigned to ST353 (1-46-3-62-2-142-3; gltA-gyrB-gdhB-recA-cpn60-gpi-rpoD) (27). Six blaOXA-23-harboring CRAB isolates exhibited an XDR phenotype. They were resistant to piperacillin, ampicillin-sulbactam, cefotaxime, ceftazidime, cefepime, aztreonam, amikacin, gentamicin, tobramycin, ciprofloxacin, and trimethoprim-sulfamethoxazole. However, susceptibility to colistin was preserved (39). Molecular analysis revealed the presence of multiple aminoglycoside-modifying enzyme genes, including aac(6′)-Ib, aac(3)-Ia, ant(3″)-Ia, and aph(3′)-Ia, in these CRAB isolates. However, the 16S rRNA methyltransferase gene armA, which confers high-level resistance to all clinically relevant aminoglycosides, was not detected (40).
Emergence of ST92 and Clonal Expansion of ST191 CRAB Strains between 2006 and 2012
CRAB isolates remained relatively uncommon at Kyungpook National University Hospital until 2006. Seven blaOXA-23- harboring CRAB isolates identified in 2006 were assigned to ST92 (1-3-3-2-2-7-3) according to the Oxford MLST scheme. This ST was originally designated ST22 before its reclassification (41). No MBL genes were detected in these CRAB isolates. Between 2004 and 2006, only a limited number of clonal lineage were identified in blaOXA-23-harboringCRAB isolates at the hospital, primarily ST92 and ST353 (27, 41). All ST92 CRAB isolates exhibited an XDR phenotype, showing resistant to at least one agent in all but two or fewer antimicrobial categories. Notably, resistance to colistin and polymyxin B was observed in ST92 CRAB isolates (27).
No epidemiological data on CRAB isolates were available from the study hospital for 2007 and 2008. However, by 2008, blaOXA-23-harboring CRAB strains belonging to ST75 (1-3-3-2-2-11-3), ST137 (1-3-3-2-2-12-3), and ST138 (1-3-3-2-2-50-3) emerged and spread nationwide in Korea (42, 43). At the study hospital, the prevalence of carbapenem resistance among A. baumannii isolates increased sharply from 40% between 2009 and 2010 to approximately 80% between 2011 and 2012 (25, 27). A total of 188 blaOXA-23-harboring CRAB isolates collected between 2009 and 2012 were analyzed using the Oxford MLST scheme. These isolates were assigned to only four STs: ST191 (n = 174, 92.6%), ST208 (n = 12, 6.4%), ST353 (n = 1, 0.5%), and ST436 (n = 1, 0.5%) (25, 27). According to the Pasteur MLST scheme, all CRAB isolates were classified as ST2, highlighting the predominance of IC2. ST191 (1-3-3-2-2-94-3), ST208 (1-3-3-2-2-97-3), and ST436 (1-3-3-2-2-103-3) represent gpi variants of ST92, indicating that they belong to the same clonal complex (CC208) and likely originated from a common ancestor. During this period, CC208 became the dominant clonal lineage among blaOXA-23-harboring CRAB isolates in Korea, with ST191 emerging as the most prevalent genotype nationwide (44). Additionally, other CRAB isolates belonging to ST75, ST138, ST357 (1-12-3-2-2-145-3), ST369 (1-3-3-2-2-106-3), and ST423 (1-46-3-2-2-142-3), which are either single locus or double locus variants of ST191, were also detected in various Korean hospitals during this time (44). Despite this genetic diversity, nearly all CRAB isolates across these STs shared a common XDR phenotype, with consistent susceptibility to colistin and polymyxin B. No resistance to tigecycline was observed during this period.
Clonal Dynamics of blaOXA-23-Harboring CRAB Strains between 2013 and 2018
A total of 243 blaOXA-23-harboring CRAB isolates collected from Kyungpook National University Hospital between 2013 and 2018 were analyzed using the Oxford MLST scheme (24, 26). Eight distinct STs were identified: ST191 (n = 55, 22.6%), ST208 (n = 48, 19.6%), ST229 (n = 39, 16.0%), ST357 (n = 17, 7.0%), ST369 (n = 39, 16.0%), ST451 (n = 34, 14.0%), ST784 (n =10, 4.1%), and ST552 (n = 1, 0.4%). Six of these STs—-ST191, ST208, ST357, ST369, ST451 (1-3-3-2-2-142-3), and ST784 (1-3-3-2-2-107-3)—belong to CC208, while ST229 (1-15-2-28-1-107-32) is part of CC110. ST552 (21-35-2-28-1-145-4) appeared only once and was considered a sporadic strain. Overall, CC208 accounted for 83.5% (203/243) of all isolates during this period. Within CC208, ST191, ST369, ST451, and ST784 were identified as gpi variants of ST208, whereas ST357 was a gyrB and gpi double locus variants of ST208. These findings underscore the role of allelic variation, particularly in the gpi gene, in driving clonal diversification.
A marked shift in the distribution of STs occurred over these six years. ST229 was initially identified as a sporadic case in 2013 but showed a steady increase in prevalence, accounting for 6.3% of isolates in 2017 and rising to 19.4% in 2018. In contrast, previously dominant clones declined in frequency: ST191 decreased from 22.6% overall to 3.2% in 2018, and ST208 decreased from 25% in earlier years to just 6.5% in 2018. ST784 appeared in 2016 but declined by 2018, while ST357 was only detected between 2013 and 2015. Meanwhile, ST369 was first identified in 2013 and became the most prevalent clone by 2018. ST451 emerged in 2016 and was the dominant lineage between 2016 and 2018. These trends indicate a dynamic clonal replacement, with older clones such as ST191 and ST208 being supplanted by newer clones like ST369 and ST451.
These local trends were consistent with findings from other Korean hospitals. In a study of 164 CRAB isolates from bacteremic patients at a tertiary hospital in Seoul collected between December 2012 and December 2018, ST191 was the most prevalent (48.8%), followed by ST451 (14.0%), ST784 (13.4%), ST369 (8.5%), ST357 (6.7%), and other STs (8.5%) (45). Another study involving 181 A. baumannii isolates, including 160 CRAB, collected from bloodstream infections between May 2016 and April 2017 across six general hospitals found that 84.5% of isolates belonged to IC2, with ST191 (n = 73), ST784 (n = 31), and ST451 (n = 29) as the predominant STs (46). Similarly, an analysis of 93 CRAB isolates from bacteremic patients between January 2009 and July 2015 at a tertiary hospital in Seoul identified ten STs, with ST191 consistently present throughout the study period, and ST208, ST369, and ST357 mainly isolated between 2009 and 2012. ST451 and ST784 emerged between 2013 and 2015 (44). These findings suggest that CRAB strains belonging to ST191, ST451, and ST784 are persistently circulating among Korean hospitals, while ST208 and ST229 may represent hospital-specific lineages at Kyungpook National University Hospital.
All CRAB isolates during this period exhibited an XDR phenotype regardless of STs. Over 90% of CRAB isolates were resistant to aminoglycosides, β-lactams, β-lactams/β-lactamase inhibitors, fluoroquinolones, cephalosporins, and folate pathway inhibitors (24, 26). However, resistance to colistin and tigecycline remained below 10% during this period. Resistance to tigecycline was first detected in ST191 and ST208 CRAB isolates in 2013 (26), and subsequently emerged in ST369, ST451, and ST784 (24). Interestingly, antimicrobial susceptibility varied among clones. For example, ST369 isolates exhibited comparatively lower resistance to aminoglycosides and trimethoprim-sulfamethoxazole, whereas ST208 isolates exhibited higher resistance to minocycline (24). Resistance to rifampin was observed in over 75% of CRAB isolates, with all ST229 CRAB isolates resistant. All CRAB isolates collected from 2009 to 2018 harbored the ISAba1-blaOXA-23 genetic structure. The 16S rRNA methyltransferase gene armA, which confers high-level resistance to aminoglycosides, was detected in ≥80% of ST191, ST208, ST451, and ST784 CRAB isolates, but in <40% of ST369 CRAB isolates (24).
CONCUSIONS
The production of OXA-23 oxacillinase has remained the primary mechanism of carbapenem resistance in A. baumannii isolates in Korea over the past two decades. At Kyungpook National University Hospital, blaOXA-23-harboring CRAB strains were first identified in 2004 and have since shown a progressive increase in prevalence. This 15-year longitudinal study revealed a clear pattern of clonal replacement, with earlier dominant clones such as ST92 and ST191 being gradually supplanted by emerging lineages, including ST369 and ST451. These shifts underscore the dynamic mature of CRAB evolution, largely driven by allelic variation of the gpi gene. Despite the high prevalence of XDR phenotypes among CRAB isolates, variations in antimicrobial susceptibility and resistance gene profiles were observed across different STs. For example, ST369 isolates exhibited comparatively lower resistance to certain agents, while ST208 and ST229 were associated with higher resistance to minocycline and rifampin, respectively. The presence of the ISAba1-blaOXA-23 structure in all isolates and the variable distribution of armA further highlight the genetic complexity underlying CRAB resistance. Importantly, this study focused on CRAB isolates collected before the COVID-19 pandemic. Since the onset of the pandemic, many countries reported increased incidence and outbreaks of CRAB, particularly among COVID-19 patients in intensive care units. The pandemic likely exacerbated the evolution of CRAB clones, potentially altering their antimicrobial susceptibility profiles and epidemiology. Therefore, robust surveillance and stringent infection control measures are essential to address and mitigate the ongoing threat of CRAB in post-COVID-19 healthcare settings.
