LAUSR

Simple Summary Otitis externa (OE) is a common disease in dogs and can be induced by various causes. After the primary causes that induced the ear canal issue, microbial infections occur secondly. As the main treatment strategies are primary cause correction and antibiotic administration, prolonged treatment is likely to induce the emergence of antibiotic resistance bacteria. Here, we describe the Enterococcus bacteria, one of the main infection agents of OE. The bacterial genus showed several species distributions and antibiotic resistance. This fact clarifies the importance of appropriate antibiotic selection and prudent antibiotic administration. As companion animals share lots of space with humans, pathogen transmissions between humans and companion animals are likely to occur. This study contributes not only to treatment strategies for Enterococcus infections but can also be used as a comparable index of antibiotic resistance of Enterococcus in the future. Abstract Otitis externa, a common disease in dogs, has different etiologies. Enterococcus is a Gram-positive bacterium that frequently causes opportunistic ear infections. Here, we determined the distribution of Enterococcus in canine otitis externa via time-of-flight mass spectrometry and biochemical tests and evaluated their resistance patterns to 10 commonly used antibiotics. Among the 197 Enterococcus isolates, E. faecalis (48.7%; 96/197) was the most common, followed by E. faecium (21.3%; 42/197), E. casseliflavus (11.7%; 23/197), E. hirae (10.7%; 21/197), E. avium (3.6%; 7/197), E. gallinarum (2.5%; 5/197), E. canintestini (1.0%; 2/197), and E. durans (0.5%; 1/197). All isolates were tested for antibiotic resistance using the Kirby–Bauer disc diffusion method. Enterococcus faecalis strains were highly resistant to erythromycin (45.8%) and rifampin (34.3%) but were generally susceptible to penicillin class antibiotics. In contrast, E. faecium isolates were highly resistant to penicillin class antibiotics (ampicillin, 61.9%; penicillin, 71.4%). Most importantly, E. faecium demonstrated high resistance to most of the antibiotics used in this study. Multidrug resistance was found in 28.4% of the isolates (56/197). This study shows prevalence and antibiotics resistance profiles of Enterococcus species in canine chronic otitis externa. The results can contribute to establish therapeutic strategies of Enterococcus infections and be used as a comparable index of antibiotic resistance of Enterococcus in the future.