Antimicrobial-Resistant Enteric Gram-Negative Bacteria Isolated from a Fatal Diarrhea in a Horse: Genomic Characterization of CTX-M-2-Producing Escherichia coli

<b>Background:</b> Infections caused by antimicrobial-resistant bacteria are difficult to treat and increase the risk of death in animals. This report describes a fatal case of diarrhea in a horse that, despite intensive treatment including surgery and broad-spectrum antimicrobials (ceftiofur and amikacin), experienced a worsening of its condition and subsequent death. <b>Methods:</b> A fecal swab sample was subjected to microbiological culture for the identification of bacteria and assessment of their phenotypical antimicrobial susceptibility profiles using the disk-diffusion and broth microdilution methods. The double-disk synergy test, polymerase chain reactions for the detection of genes encoding extended-spectrum β-lactamases, and whole-genome sequence-based analysis were also performed. <b>Results:</b> Strains of <i>Salmonella enterica</i> and <i>Escherichia coli</i> were isolated, with the <i>E. coli</i> strain DSL-HVUVV-2025 presenting resistance to a third-generation cephalosporin. Accordingly, the <i>bla</i>_CTX-M-2 gene was identified in the DSL-HVUVV-2025 strain, which was submitted to whole-genome sequencing. Genomic analysis showed several antimicrobial resistance determinants, as well as virulence genes, including those associated with the enteroaggregative pathotype. The <i>bla</i>_CTX-M-2 gene was surrounded by an IS<i>CR1</i> element and embedded in a complex class 1 integron that is part of the Tn<i>7337</i> transposon. Strain DSL-HVUVV-2025 belonged to a novel sequence type. <b>Conclusions:</b> This case highlights the importance of monitoring antimicrobial resistance and performing genomic characterization of bacteria involved in equine diarrhea to guide effective clinical management in veterinary hospitals. It also reinforces the role of horses as potential carriers of WHO critical priority pathogens and the need for responsible antimicrobial use.