RESUMEN
BACKGROUND: The emergence and spread of mobilized colistin resistance (mcr) genes are a global health concern. OBJECTIVES: In this study, we report the detection and genomic characterization of mcr-9 in a colistin-susceptible Enterobacter hormaechei (EH23) recovered from a pediatric patient in Lebanon. RESULTS: EH23 was susceptible to colistin with a minimal inhibitory concentration (MIC) of 0.25 mg/L. Studying the mcr-9 genetic environment revealed that it was chromosomal and was bracketed by IS903 and IS26. QseCB, a two-component regulatory system, mediating the inducible expression of mcr-9 gene was not detected within the mcr-9 cassette but elsewhere on the genome. EH23 was 99.96% similar based on average nucleotide identity (ANI) to another mcr-negative E. hormaechei OIPH-N069 isolate recovered from Japan. wgSNP-based phylogenetic analysis divided all mcr-9 positive E. hormaechei isolates into five clades (I to V), with isolates from the same ST being clustered together. CONCLUSION: The silent spread of mcr-9, particularly in the globally successful ST-78 Enterobacter lineage, is worrisome and requires close monitoring in humans and animals.