Phenotypic and Molecular Characteristics of Carbapenem-Resistant Acinetobacter baumannii Isolates from Bulgarian Intensive Care Unit Patients.

Carbapenem-resistant Acinetobacter baumannii (CRAB) is designated as an urgent public health threat, both due to its remarkable multidrug resistance and propensity for clonal spread. This study aimed to explore the phenotypic and molecular characteristics of antimicrobial resistance in CRAB isolates ( n = 73) from intensive care unit (ICU) patients in two university hospitals in Bulgaria (2018-2019). The methodology included antimicrobial susceptibility testing, PCR, whole-genome sequencing (WGS), and phylogenomic analysis. The resistance rates were as follows: imipenem, 100%; meropenem, 100%; amikacin, 98.6%; gentamicin, 89%; tobramycin, 86.3%; levofloxacin, 100%; trimethoprim-sulfamethoxazole, 75.3%; tigecycline, 86.3%; colistin, 0%; and ampicillin-sulbactam, 13.7%. All isolates harbored bla OXA-51-like genes. The frequencies of distribution of other antimicrobial resistance genes (ARGs) were: bla OXA-23-like , 98.6%; bla OXA-24/40-like , 2.7%; armA , 86.3%; and sul1 , 75.3%. The WGS of selected extensively drug-resistant A. baumannii (XDR-AB) isolates ( n = 3) revealed the presence of OXA-23 and OXA-66 carbapenem-hydrolyzing class D β-lactamases in all isolates, and OXA-72 carbapenemase in one of them. Various insertion sequencies, such as IS Aba24 , IS Aba31 , IS Aba125 , IS Vsa3 , IS 17 , and IS 6100 , were also detected, providing increased ability for horizontal transfer of ARGs. The isolates belonged to the widespread high-risk sequence types ST2 ( n = 2) and ST636 ( n = 1) (Pasteur scheme). Our results show the presence of XDR-AB isolates, carrying a variety of ARGs, in Bulgarian ICU settings, which highlights the crucial need for nationwide surveillance, especially in the conditions of extensive antibiotic usage during COVID-19.