Determinants of Antimicrobial Resistance in Acinetobacter baumannii Isolates from Intensive Care Patients in Latvia

Acinetobacter baumannii is a leading nosocomial pathogen in intensive care units (ICUs), often resistant to multiple antibiotics. Data from the Baltic region remain scarce, limiting infection control and stewardship strategies. We conducted an integrated phenotypic–genotypic analysis of A. baumannii isolates collected from ICU patients in a tertiary-care hospital in Latvia (July 2022–June 2024). Antimicrobial susceptibility testing was performed for major antibiotic classes, and whole-genome sequencing (WGS) was used to identify genomic resistance determinants. We analysed 52 clinical isolates from 45 ICU patients. Multidrug resistance was nearly universal (98%), with complete resistance to carbapenems, fluoroquinolones, and ciprofloxacin, and > 95% resistance to aminoglycosides and trimethoprim–sulfamethoxazole. Colistin activity was largely preserved, with resistance detected in only one isolate, despite widespread polymyxin resistance–associated mutations. Genotypic findings corroborated phenotypic results in most cases but also revealed discordance, with resistance determinants not always expressed phenotypically. All isolates belonged to the ST2 lineage, highlighting clonal homogeneity. No plasmid replicons were detected, suggesting chromosomal elements as the primary resistance drivers. Our first integrated dataset from the Baltic region demonstrates alarming resistance levels and clonal dominance of ST2. The observed genotype–phenotype discordance highlights the importance of combining WGS with susceptibility testing for accurate resistance assessment.