Genomic characterization of a Klebsiella pneumoniae ST231 clinical isolate from Tunisia co-producing mcr-1.1 and blaOXA-181

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Objective Plasmid-encoded colistin resistance mcr-1 genes are mainly reported in Escherichia coli worldwide. Here, we characterized a multidrug-resistant Klebsiella pneumoniae 70241 clinical isolate harbouring mcr-1.1 and bla OXA-181 , recovered from a tertiary-care hospital in Tunis, Tunisia. Methods Antimicrobial susceptibility was tested by broth microdilution and interpreted using EUCAST 2025 criteria. Whole-genome sequencing was performed with Oxford Nanopore long-read technology. In silico analyses (MLST, resistance genes, plasmid typing) were conducted using tools from the Center for Genomic Epidemiology, while comparative plasmid analysis was performed using BLAST and PROKSEE. Conjugation assays assessed plasmid transferability. Results K. pneumoniae 70241 belonged to ST231 and showed a multidrug-resistant phenotype with low-level colistin resistance (MIC = 4 µg/ml). The mcr-1.1 gene was carried on a 33.3-kb IncX4 plasmid closely related to animal- and food-associated plasmids. The bla OXA-181 gene was located within a truncated IS 26 -based Tn 6361 -like transposon on a 49.4-kb IncX3 plasmid also harboring qnrS1 . Additional plasmids, a fused IncR–IncFIA(HI1) carrying qnrB6, qacE and sul1 and a large IncFIB(K) carrying bla SHV-1 and tet(D) , were also found. The mcr-1.1 harbouring IncX4 plasmid was transferred at high frequency (4 × 10 -2 ) to E. coli J53, confirming its self-transmissibility. Conclusion This study describes the first genomic features of a K. pneumoniae ST231 clinical isolate from Tunisia co-producing Mcr-1.1 and OXA-181. The presence of highly transmissible IncX4 and IncX3 plasmids highlights the risk of horizontal spread of last-resort resistance genes across clinical and animal settings, emphasizing the need for integrated genomic surveillance within a One Health framework.

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