Cultivable Otic Microorganisms and Antimicrobial Resistance Profiles in Dogs and Cats with Otitis Externa

Background/Objectives: Feline otitis externa is less well characterized microbiologically than the canine disease. This retrospective laboratory-based study described cultivable otic microorganisms and antimicrobial resistance among isolates from 112 animals with otitis externa (46 dogs and 66 cats). Methods: A total of 204 archived isolates were identified to species level by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Bacterial isolates underwent disk-diffusion antimicrobial susceptibility testing with organism-specific panels. Record-based multiple antibiotic resistance (MAR) indices and an organism-specific exploratory multidrug resistance (MDR) category assessment were calculated. Phenotypically methicillin-resistant staphylococci were tested for mecA by PCR. Results: The collection comprised 188 bacterial and 16 yeast isolates. Staphylococcus pseudintermedius was the most frequent (30/204; 14.7%), followed by Staphylococcus aureus (21/204; 10.3%), Escherichia coli (20/204; 9.8%), Pseudomonas aeruginosa (17/204; 8.3%), and Proteus mirabilis (15/204; 7.4%). Of 181 bacterial isolates with AST records, 171 had at least one MAR-eligible result; mean record-based MAR indices were 0.341 in canine isolates (n = 88) and 0.343 in feline isolates (n = 83). Heterogeneous panels and incomplete interpretative provenance precluded a standardized pooled MDR estimate. mecA was detected in 19/28 (67.9%) phenotypically methicillin-resistant staphylococci, including 8/12 (66.7%) S. pseudintermedius and 11/15 (73.3%) S. aureus isolates. Conclusions: This study documented diverse cultivable otic microorganisms, species-dependent antimicrobial resistance profiles, and mecA-positive staphylococci among archived isolates from dogs and cats with otitis externa. These findings contribute to the characterization of antimicrobial resistance in companion-animal otic isolates, while requiring cautious interpretation in relation to topical treatment.