Antimicrobial resistance and molecular characterization of extended-spectrum β-lactamase/AmpC β-lactamase–producing Proteus mirabilis isolates from companion dogs in South Korea (2018–2024)

Objective: To investigate antimicrobial resistance profiles and molecular characteristics of third-generation cephalosporin- and cephamycin-resistant Proteus mirabilis isolated from clinically ill dogs across South Korea from 2018 through 2024. Methods: Antimicrobial susceptibility of isolates was assessed by broth microdilution. Molecular characteristics were determined by polymerase chain reaction and multilocus sequence typing. Results: In total, 667 P mirabilis were isolated from skin/ear, feces, and urine/reproductive systems of companion dogs. Among them, 27.3% of isolates exhibited a multidrug-resistant (MDR) phenotype. Molecular characterization of 38 extended-spectrum β-lactamase (5.7%)/AmpC β-lactamase-carrying strains revealed a high prevalence of β-lactamase cephamycinase-2 gene blaCMY-2 (65.8%) and β-lactamase cefotaximase gene blaCTX-M variants, including blaCTX-M-65, blaCTX-M-15, and blaCTX-M-90. Multilocus sequence typing identified 16 distinct sequence types (STs), with ST135 (39.5%) and ST212 (21.1%) emerging as the most prevalent lineages. Phylogenetic analysis revealed that the first major cluster contains 31 isolates (81.6%) across 10 STs centered around ST135, whereas the second cluster comprises 7 isolates (18.4%) across 6 STs centered on ST292. Notably, the ST135-centered cluster was associated with robust MDR phenotypes. Conclusions: These findings indicate that companion dogs harboring MDR P mirabilis are among several potential reservoirs within the One Health context. Especially, clone ST135 may serve as a primary evolutionary axis and a high-risk endemic clone within the Korean canine population. Clinical Relevance: Our study highlights the critical role of dominant lineages in dissemination of extended-spectrum β-lactamase/AmpC β-lactamase genes and emphasizes the urgent need for continuous One Health surveillance to monitor potential transmission of these clones between companion animals and humans.