Genomic characterization of carbapenem-resistant Acinetobacter baumannii from a specialized orthopaedic hospital in Southeast China, with phenotypic analysis of biofilm formation

Tengfei Shi, Shaohan Xu, X B Zheng, Weiqiang Shi, Rui Zhang, H Z Chen, H Z Chen, Tengyi Chen, H Z Chen, H Z Chen

Background Carbapenem−resistant Acinetobacter baumannii (CRAB) is a major threat to hospitalised patients, particularly in intensive care units and orthopaedic wards where implant−associated infections are common. However, the genomic and biofilm characteristics of CRAB in orthopaedic specialty hospitals remain poorly understood. Methods A total of 97 non-duplicate CRAB isolates collected between 2024 and 2025 were included. Antimicrobial susceptibility testing was performed using the VITEK-2 system, and biofilm formation was quantitatively assessed by the crystal violet method. Whole-genome sequencing (WGS) was carried out on the Illumina platform to analyse multilocus sequence typing (MLST), capsular types, resistance genes and virulence genes. A phylogenetic tree was constructed based on single-nucleotide polymorphisms (SNPs). Results ST2 was the dominant clone (95.88%) among the 97 isolates, and KL3 was the most prevalent capsular type (83.51%). All isolates carried intrinsic blaOXA-51-like genes, predominantly blaOXA-66 (95.88%). The most common acquired carbapenemase gene was blaOXA-23 (98.97%), and two isolates carried metallo-β-lactamase (MBL) genes ( blaNDM-1 and blaNDM-5 , respectively). All isolates exhibited a multidrug-resistant phenotype, with low resistance rates to tigecycline (6.19%) and minocycline (7.22%), and all remained susceptible to colistin. Strong biofilm formers accounted for 91.75% of isolates, and the carriage rates of biofilm-associated genes ( bap , csuABCDE , pgaABCD ) exceeded 90%. Phylogenetic analysis grouped the isolates into three clonal clades, with the majority (88.66%) falling into Clade C (ST2/KL3). This clade had been circulating in China as an outbreak lineage since 2018, gradually replacing Clade B (ST2/KL2), and became the dominant clone in 2024–2025. Conclusion CRAB isolates in this orthopaedic specialty hospital are dominated by the ST2/KL3 clone, which carries multiple resistance and virulence genes, exhibits a remarkably strong biofilm-forming ability, and shows a capsular switch trend from KL2 to KL3. Enhanced molecular surveillance of this dominant clone and increased attention to anti-biofilm strategies for orthopaedic implant-related infections are strongly recommended.