Silent carriage of mcr-9 on IncHI2 plasmid in an Enterobacter hormaechei strain causing a urinary tract infection in a dog from Portugal.

The emergence of plasmid-mediated colistin resistance genes (mcr genes) poses a major threat to public health. Among these, the mcr-9 gene is frequently detected without conferring phenotypic resistance. An mcr-9-positive Enterobacter hormaechei isolated from a canine urinary tract infection in Portugal was characterized by whole-genome sequencing, revealing a multidrug-resistant IncHI2 plasmid carrying the mcr-9 gene. This plasmid showed structural similarity to other publicly available plasmid sequences from human and animal sources worldwide. The strain harbored a conserved genetic region composed of the nickel/copper-associated operon rcnR-rcnA-pcoE-ISSgsp1-pcoS-IS903-mcr-9-wbuC, which is involved in metal homeostasis and copper tolerance under anaerobic conditions. Antimicrobial susceptibility testing revealed colistin susceptibility. Notably, the regulatory genes qseC and qseB, which have been implicated in the activation of mcr-9 expression, were absent, potentially explaining this phenotype. These findings highlight the silent dissemination potential of mcr-9 in companion animals and reinforce the importance of genomic surveillance under a One Health framework.