Phenotypic antimicrobial susceptibility and genomic resistance determinants in canine otitis-associated Pseudomonas aeruginosa isolates

BACKGROUND: Pseudomonas aeruginosa is a clinically important opportunistic pathogen in canine otitis externa, particularly in chronic, recurrent or treatment-refractory cases. Its low outer-membrane permeability, biofilm-forming capacity, efflux systems and ability to acquire additional resistance determinants complicate empirical therapy and make local surveillance essential. METHODS: This study integrated broth microdilution-based minimum inhibitory concentration (MIC) testing with whole-genome analysis to characterize antimicrobial susceptibility patterns and genomic resistance determinants in canine otitis-associated P. aeruginosa isolates from Hungary. A total of 110 isolates were tested against 14 antimicrobial or antiseptic agents. Official clinical breakpoints and European Committee on Antimicrobial Susceptibility Testing (EUCAST) epidemiological cut-off values were applied where available, while agents lacking validated interpretive criteria were evaluated descriptively. A phenotypically selected subset of 70 isolates, chosen to cover the major MIC ranges, resistance profiles and modified MAR-index spectrum observed in the full collection, was subjected to long-read sequencing, and all 70 genome assemblies were included in comprehensive antibiotic resistance database (CARD)-based resistance gene screening, multi-locus sequence typing (MLST) and genome quality assessment. RESULTS: High frequencies of elevated MICs were detected for several clinically relevant agents. The highest proportions above interpretive thresholds were observed for enrofloxacin (109/110, 99.1%), marbofloxacin (99/110, 90.0%), piperacillin-tazobactam (75/110, 68.2%), ceftazidime (71/110, 64.5%) and tobramycin (62/110, 56.4%). The median modified multiple-antimicrobial-resistance index, calculated only from agents with interpretable resistance thresholds, was 0.56. Genomic analysis showed a broad intrinsic resistance background dominated by efflux-associated determinants and recurrent aminoglycoside, beta-lactam and fluoroquinolone-associated genes, including APH(3')-IIb, Mex-family efflux components, Opr-associated transporters, arnA, basS, PDC/OXA beta-lactamases and crpP in a subset of isolates. CONCLUSION: These findings support the need for culture- and MIC-guided therapy in canine P. aeruginosa otitis and support the value of including companion-animal clinical isolates in national and regional antimicrobial resistance surveillance frameworks.

Authors: M. A. Veres, Z. A. Tóth, E. Illés, P. Mag, E. Kaszab, E. Fehér, Á. Kerek and Á. Jerzsele Title: Phenotypic antimicrobial susceptibility and genomic resistance determinants in canine otitis-associated Pseudomonas aeruginosa isolates Full source: Front Vet Sci, 2026,Vol 13, pp 1902036
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BACKGROUND: Pseudomonas aeruginosa is a clinically important opportunistic pathogen in canine otitis externa, particularly in chronic, recurrent or treatment-refractory cases. Its low outer-membrane permeability, biofilm-forming capacity, efflux systems and ability to acquire additional resistance determinants complicate empirical therapy and make local surveillance essential. METHODS: This study integrated broth microdilution-based minimum inhibitory concentration (MIC) testing with whole-genome analysis to characterize antimicrobial susceptibility patterns and genomic resistance determinants in canine otitis-associated P. aeruginosa isolates from Hungary. A total of 110 isolates were tested against 14 antimicrobial or antiseptic agents. Official clinical breakpoints and European Committee on Antimicrobial Susceptibility Testing (EUCAST) epidemiological cut-off values were applied where available, while agents lacking validated interpretive criteria were evaluated descriptively. A phenotypically selected subset of 70 isolates, chosen to cover the major MIC ranges, resistance profiles and modified MAR-index spectrum observed in the full collection, was subjected to long-read sequencing, and all 70 genome assemblies were included in comprehensive antibiotic resistance database (CARD)-based resistance gene screening, multi-locus sequence typing (MLST) and genome quality assessment. RESULTS: High frequencies of elevated MICs were detected for several clinically relevant agents. The highest proportions above interpretive thresholds were observed for enrofloxacin (109/110, 99.1%), marbofloxacin (99/110, 90.0%), piperacillin-tazobactam (75/110, 68.2%), ceftazidime (71/110, 64.5%) and tobramycin (62/110, 56.4%). The median modified multiple-antimicrobial-resistance index, calculated only from agents with interpretable resistance thresholds, was 0.56. Genomic analysis showed a broad intrinsic resistance background dominated by efflux-associated determinants and recurrent aminoglycoside, beta-lactam and fluoroquinolone-associated genes, including APH(3')-IIb, Mex-family efflux components, Opr-associated transporters, arnA, basS, PDC/OXA beta-lactamases and crpP in a subset of isolates. CONCLUSION: These findings support the need for culture- and MIC-guided therapy in canine P. aeruginosa otitis and support the value of including companion-animal clinical isolates in national and regional antimicrobial resistance surveillance frameworks.