Resistance to Human-Reserved Antimicrobials in Clinical and Commensal Enterococcus faecalis and Enterococcus faecium from Dogs and Cats

Background/Objectives: Although companion animals are recognized as reservoirs of antimicrobial resistant enterococci, it remains unclear whether antimicrobial resistance differs between clinical and commensal isolates, particularly for antimicrobials reserved for human medicine. This study compared antimicrobial susceptibility profiles of clinical and commensal E. faecalis and E. faecium isolates from dogs and cats, focusing on antimicrobials reserved for human medicine under European regulation and EMA/AMEG classification. Methods: A total of 99 isolates (49 E. faecalis and 50 E. faecium) collected between 2022 and 2025 were analysed by broth microdilution, gradient diffusion, and disk diffusion methods. Antimicrobial resistance patterns were compared according to bacterial species and isolate origin. Results: E. faecium exhibited significantly higher resistance rates than E. faecalis, particularly to ampicillin, fluoroquinolones, erythromycin, nitrofurantoin, and vancomycin. Multidrug resistance was widespread, affecting all clinical E. faecium isolates and 72.2% of commensal E. faecium, whereas corresponding proportions in E. faecalis were 64.1% and 40.0%, respectively. Clinical E. faecium isolates showed significantly higher resistance than commensal isolates to antimicrobials commonly used in veterinary medicine, including ampicillin, fluoroquinolones, erythromycin, gentamicin, streptomycin, and tetracycline. Conversely, no significant differences were observed for antimicrobials reserved for human medicine, including linezolid, vancomycin, teicoplanin, tigecycline, daptomycin, rifampicin, and quinupristin/dalfopristin. Conclusions: The higher resistance observed in clinical isolates to veterinary antimicrobials is consistent with the selective pressure exerted by antimicrobial use in veterinary clinical settings, whereas resistance to human-reserved agents may reflect epidemiological processes that are independent of antimicrobial exposure in veterinary clinical settings. Continuous integrated One Health surveillance is essential to monitor the emergence and dissemination of these resistance phenotypes. The AMR profiles observed in companion animals further support the need for integrated One Health surveillance, combining veterinary, human, and environmental data to better understand the emergence and dissemination of resistant enterococci.