Longitudinal surveillance of molecular epidemiology and antimicrobial resistance of carbapenem-resistant Acinetobacter baumannii in Thailand

Background Carbapenem-resistant Acinetobacter baumannii (CRAB) is a major cause of healthcare-associated infections, especially in intensive care units. Its rapid acquisition of antimicrobial resistance necessitates molecular epidemiology to inform infection control measures and treatment strategies. This study evaluated antimicrobial resistance phenotypes and the prevalence of resistance genes among clinical CRAB isolates in Thailand, complemented by MLST-based population structure analysis incorporating a temporal dimension of clonal distribution compared with previous reports. Methods This study analyzed 316 clinical A. baumannii isolates from five regions across Thailand (2022–2023). Species confirmation was performed by PCR targeting bla OXA-51–like . Antimicrobial susceptibility testing against thirteen antibiotics followed CLSI 2024 guidelines. Resistance genes were identified by PCR, and MLST with PHYLOViZ goeBURST characterized population structure. Results All isolates demonstrated carbapenem resistance, with the majority exhibiting extensively drug-resistant (XDR) phenotypes. Isolates showed resistance to β-lactams, fluoroquinolones, aminoglycosides, tetracyclines, and sulfamethoxazole/trimethoprim, while remaining susceptible to minocycline. MLST revealed ST2 as the predominant clone (78.8%), followed by ST164 (8.9%) and ST16 (6.0%). Class A β-lactamase bla VEB was highly detected (95.3%), followed by Class D β-lactamase bla OXA-23-like (93.7%). Temporal distribution of clonal lineages revealed persistence of major STs without clonal shifts, with ST2, ST164, and ST25 remaining dominant across sampling periods. ST164 (IC11) notably exhibited high resistance gene carriage capacity. Conclusions The results reinforce the predominance and persistence of ST2 (IC2) in the CRAB population circulating in Thailand and geographic dissemination of high-risk clones across different regions. This highlights the importance of continuous molecular surveillance to guide infection prevention and antimicrobial stewardship.