Phenotypic Susceptibility, Resistance-Associated Genomic Determinants, and Mobile-Genetic-Element Context of Canine Otitis Externa-Associated Pseudomonas aeruginosa Collected in Hungary in 2010 and 2017

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Background: Canine otitis externa caused by Pseudomonas aeruginosa is frequently chronic and treatment refractory, yet integrated historical phenotype–genotype data remain limited. We characterized an archival Hungarian collection obtained in 2010 and 2017. Methods: Broth microdilution minimum inhibitory concentrations (MICs) were determined for 67 isolates against 13 antimicrobial agents and chlorhexidine, and 59 isolates had matched long-read whole-genome assemblies. Resistance-associated determinants were identified using the Comprehensive Antibiotic Resistance Database, and their predicted proximity to mobile genetic elements and plasmid-like contigs was evaluated. Results: Current canine Clinical and Laboratory Standards Institute breakpoints classified 59/67 isolates (88.1%) as enrofloxacin resistant and 43/67 (64.2%) as marbofloxacin resistant. European Committee on Antimicrobial Susceptibility Testing epidemiological cut-off values identified non-wild-type subsets for ciprofloxacin (7/67), tobramycin (4/67), imipenem (1/67), and piperacillin–tazobactam (51/67), whereas all isolates remained within the available wild-type distributions for ceftazidime, gentamicin, and amikacin. No continuous MIC distribution changed significantly between sampling years after false-discovery-rate correction, although categorical marbofloxacin resistance decreased from 87.5% in 2010 to 42.9% in 2017. Resistance-gene screening identified 64 unique determinants, including near-universal efflux-system components, PDC and OXA-50-like β-lactamases, APH(3′)-IIb, arnA, and basS. The fluoroquinolone-associated determinant crpP occurred in 37/59 genomes and was located on plasmid-predicted contigs in six isolates. No determinant–MIC association remained significant after correction for multiple testing. Conclusions: The discordance between conserved resistance-gene repertoires and heterogeneous MICs demonstrates that sequence-based detection alone is insufficient for therapeutic inference. Integrated phenotypic and genomic surveillance is therefore required to support evidence-based antimicrobial stewardship in canine pseudomonal otitis.

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