Prevalence and Antimicrobial Resistance of Staphylococcus aureus and Staphylococcus schleiferi Isolated from Dogs with Otitis Externa and Healthy Dogs.

Background/Objectives: Antimicrobial resistance (AMR) in companion animals is a growing One Health concern due to the close interaction between pets and humans. Staphylococcus aureus (S. aureus) and Staphylococcus schleiferi (S. schleiferi) are common colonizers of the canine ear canal and can act as reservoirs of resistance. This study aimed to assess the prevalence and antimicrobial resistance profiles of S. aureus and S. schleiferi isolated from dogs with otitis externa and clinically healthy dogs in western Romania. Methods: A total of 973 canine ear swabs were collected, 503 from dogs with otitis externa and 470 from healthy dogs. Isolates were identified using MALDI-TOF MS. Antimicrobial susceptibility testing was performed using the VITEK® 2 Compact system, bioMérieux, Marcy-l'Étoile, France, and interpreted according to CLSI VET01 guidelines, with 13 antimicrobials representing multiple drug classes. Results: S. aureus was more prevalent in healthy dogs (20%) than in otitis cases (4%), while S. schleiferi was more common in otitic samples (7.5% vs. 4%). Among S. aureus isolates from otitic dogs, penicillin resistance was highest (65%), and 25% were multidrug-resistant (MDR). In healthy dogs, S. aureus showed 54.3% penicillin resistance and 16% MDR prevalence. Four MRSA strains (4.3%) were identified only in healthy dogs. S. schleiferi exhibited the highest resistance to clindamycin, with MDR rates of 10.6% in otitic and 5.6% in healthy dogs. No MRSS strains were detected. Conclusions: Clinically healthy dogs may serve as asymptomatic carriers of resistant Staphylococcus strains, including MRSA. Routine antimicrobial susceptibility testing is essential to inform treatment choices and mitigate resistance dissemination within veterinary and public health contexts.