Carbapenem-Resistant Acinetobacter baumannii in Zagreb, Croatia, in Post-COVID-19 Pandemic Period: Resistance Trends and Mechanisms

During the COVID-19 pandemic carbapenem-resistant Acinetobacter baumannii (CRAB) was found to be the major pathogen associated with ventilator-associated pneumonia in mechanically ventilated patients. This prompted us to analyze the post-pandemic mechanisms of carbapenem resistance, antibiotic resistance trends, and molecular epidemiology of CRAB in Croatia. In total, 94 CRAB isolates from two hospital centers, including outpatient settings, were investigated. Antimicrobial susceptibility testing was performed by broth microdilution. PCR was used to detect genes encoding carbapenemases of group A, B and D and extended-spectrum β-lactamases (ESBL). Randomly selected isolates were subjected to whole resistome analysis by Inter-array CarbaResist Kit and whole-genome sequencing (WGS). Phylogenetic tree and sequence types (STs) were retrieved from WGS. Plasmid incompatibility groups were determined by PCR-based replicon typing (PBRT). All isolates were extensively drug resistant (XDR), showing resistance to ceftazidime, cefepime, piperacillin–tazobactam, imipenem, meropenem, gentamicin, amikacin and ciprofloxacin, and 13% (n = 12) were also resistant to colistin. The Hodge and CIM test exhibited poor sensitivity with only 32 and 30% of isolates being identified as carbapenemase producers, respectively. PCR identified blaOXA-23 as the dominant carbapenemase gene in both hospitals, found in 71% of the isolates (67/94). In an outpatient setting, blaOXA-24/40 was dominant. blaOXA-23 and blaOXA-72 were the only allelic variants. The Inter-array CarbaResist Kit and whole-genome sequencing (WGS) identified a variety of aminoglycoside (armA, ant(3″)-IIa, aph(3″)-Ib, aph(6)-Id) and sulphonamide resistance (sul1 and sul2) genes. The representative blaOXA-23-positive isolates belonged to ST2, while blaOXA-72-positive isolates were allocated to ST492. These data show that there are different populations of XDR A. baumannii between hospital and outpatients.