The evolution of the multidrug-resistant and globally distributed ST2 Acinetobacter baumannii in China

Emergence and global expansion of the multidrug-resistant (MDR) clones of Acinetobacter baumannii which often cause difficult-to-treat infections, have been increasingly reported in nosocomial and community-acquired infections. However, as the most widely prevalent sequence type of MDR A. baumannii, the distribution and evolution of ST2 A. baumannii remain obscure. In this study, a multicenter epidemiological survey was conducted using A. baumannii isolates obtained from 27 provinces and municipalities across China between 1999 and 2022. It was observed that ST2 was the most common sequence type among the A. baumannii isolates in China. Subsequently, 143 representative ST2 isolates were selected for genomic and evolutionary analyses. Out of these, 23 isolates were collected in this study, while an additional 120 isolates were downloaded from the GenBank database. We demonstrated that ST2 A. baumannii strains had diverse phylogenetic structures with nine sequence clusters (SCs). Extensive recombination events related to capsular polysaccharides and phages were detected, enabling distinction between different SCs. A high number of antibiotic resistance genes (ARGs) (Median 30 [range 22 to 36]) were identified, with 23 strains in this study showing pandrug resistance. All 143 ST2 strains were found to carry genes associated with antibiotic efflux pumps, such as amvA, adeGH and adeIJK, abeS, and abeM, with a most recent common ancestor (MRCA) arose around 1976. Antimicrobial susceptibility testing (AST) showed that 23 isolates collected in this study were resistant to meropenem. Extensive presence of A. baumannii genomic resistance islands (AbGRIs), including AbGRI1, AbGRI2, and AbGRI3, was observed within ST2 A. baumannii, with multiple resistance genes co-located. This study demonstrated the homologous recombination and antibiotic resistance contribute to evolution of ST2 A. baumannii, with a MRCA arose around 1976 in China. The results are highly relevant for enhancing the understanding of ST2 A. baumannii diversity and evolutionary pathways among practitioners, researchers and policymakers, informing infection reduction strategies.