Genomic analysis of carbapenem resistant Acinetobacter baumannii outbreak in a burn intensive care unit of a tertiary-care hospital in Madrid, Spain

Patients in burn intensive care units (BICUs) are at high risk of infections caused by multidrug-resistant pathogens, which can lead to hospital outbreaks. To investigate an outbreak caused by carbapenem-resistant Acinetobacter baumannii (CRAB) in a BICU at a tertiary care hospital in Madrid, Spain, using whole-genome sequencing (WGS) and antibiotic susceptibility testing. CRAB isolates were obtained from rectal swabs collected during routine epidemiological screening of BICU patients, as well as clinical and environmental samples collected throughout the outbreak. The genomes of the isolates were sequenced using Ion Torrent™ and MinION™ technologies. Antibiotic susceptibility testing was conducted by microdilution using the MicroScan NF50 panel. Susceptibility to cefiderocol was tested using disk diffusion and broth microdilution (ComASP®). The outbreak occurred between October 2022 and August 2023, affecting 26 patients (median age: 56 years, IQR 34–70). Genomic analysis identified the isolates as belonging to ST2/ST451 (Pasteur/Oxford schemes), carrying blaOXA−23 carbapenemase gene. Environmental sampling detected CRAB on 20 of 105 tested surfaces. The outbreak's genomic evolution included the emergence of hypermucoid isolates and blaADC−73 mutants with increased cefiderocol minimum inhibitory concentrations (MICs). Novel mutations and combinations in blaADC−73 were observed, likely selected under cefiderocol pressure. Antibiotic susceptibility testing using the microdilution method was more sensitive in detecting these mutations. The outbreak ultimately necessitated the closure of the BICU for disinfection with high-concentration H2O2. WGS enabled detailed tracking of CRAB's genomic evolution during the outbreak, identifying mutations associated with increased cefiderocol MICs. This information supported effective infection control measures, highlighting the utility of WGS in managing hospital outbreaks.