In vivo evolution of tigecycline resistance in ST540 carbapenem-resistant Acinetobacter baumannii: Mechanisms and global epidemiological perspective.

Carbapenem-resistant Acinetobacter baumannii (CRAB) is a persistent nosocomial pathogen, posing a major global health threat owing to limited treatment options. Although tigecycline is still effective against CRAB, resistance emergence has become a critical concern. This study aimed to elucidate the in vivo evolutionary mechanisms underlying tigecycline resistance in CRAB. A total of 11 STpas2-SToxf540 CRAB strains were recovered from rectal swabs, sputum samples, and the surrounding environmental specimens of an intensive care unit (ICU) hospitalized patient who had received tigecycline treatment. CgSNP analysis confirmed environment-to-patient transmission among these CRAB isolates. Comparative genomic analysis indicated that mutations in adeS, pgaA, and gbsA may be associated with tigecycline resistance. In situ mutagenesis and antimicrobial susceptibility testing verified that the adeS N125S mutation mediates tigecycline resistance and collateral sensitivity to cefoperazone/sulbactam. Growth curve assays demonstrated that the adeS N125S mutation imposes a fitness cost on CRAB. Transcriptional analysis showed that the adeS N125S mutation drives the development of tigecycline resistance by upregulating the expression of the AdeABC efflux pump. By examining 45,377 publicly available global A. baumannii genome sequences, we characterized the molecular epidemiology of the STpas2-SToxf540 lineage and the distribution of adeS mutations. Our findings underscore the need to strengthen surveillance and rational antimicrobial use to prevent CRAB dissemination and antimicrobial resistance evolution, particularly in ICUs.