Rare ST124 and ST586 lineages co-circulate with the dominant ST2 clone among extensively drug-resistant Acinetobacter baumannii in Iran

Abstract Carbapenem-resistant Acinetobacter baumannii (CRAB) poses a critical global threat, necessitating precise molecular surveillance. This study investigated the clonal diversity and resistance mechanisms of A. baumannii in Northwestern Iran. A total of 89 non-duplicate isolates were collected from ICU patients (May 2024–June 2025). Susceptibility testing was performed per CLSI guidelines, and resistance determinants ( bla OXA−23 , bla PER , bla TEM , etc.) were screened by PCR. Clonal lineages were identified using Pasteur Multi-Locus Sequence Typing (MLST). The isolates exhibited alarming resistance rates, with 95.5% classified as CRAB and 91% as extensively drug-resistant (XDR). The bla OXA−23 carbapenemase was the predominant resistance driver (95.5%). MLST analysis of representative isolates identified ST2 (12/16, 75%) as the dominant lineage strictly within the purposively selected subset. Significantly, this study provides the first report of rare sequence types ST124 and ST586 in Iran. While bla OXA−23 -producing ST2 remains the primary endemic threat, the emergence of these rare singletons acts as an early warning indicator for the introduction of previously undetected genetic reservoirs. Colistin retained the highest in vitro activity (97.8% intermediate/colistin-active), emphasizing its crucial role as a last-resort therapy.