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.