| Summary: | <i>Mannheimia haemolytica</i> is the main pathogen contributing to pneumonic pasteurellosis in sheep. The aim of this study was to investigate the antimicrobial resistance levels in <i>M. haemolytica</i> isolates from the lungs of slaughtered sheep and to examine the genetic resistance mechanisms involved. A total of 256 <i>M. haemolytica</i> isolates, 169 from lungs with pneumonic lesions and 87 from lungs without lesions, were analyzed by the disk diffusion method for 12 antimicrobials, and the whole genome of 14 isolates was sequenced to identify antimicrobial resistance determinants. Levels of phenotypic resistance ranged from <2% for 10 antimicrobials (amoxicillin, amoxicillin-clavulanic, ceftiofur, cefquinome, lincomycin/spectinomycin, gentamicin, erythromycin, florfenicol, enrofloxacin, and doxycycline) to 4.3% for tetracycline and 89.1% for tylosin. Six isolates carried <i>tetH</i> genes and four isolates carried, in addition, the <i>strA</i> and <i>sul2</i> genes in putative plasmid sequences. No mutations associated with macrolide resistance were identified in 23 rDNA sequences, suggesting that the <i>M. haemolytica</i> phenotypic results for tylosin should be interpreted with care in the absence of well-established epidemiological and clinical breakpoints. The identification of strains phenotypically resistant to tetracycline and of several resistance genes, some of which were present in plasmids, highlights the need for continuous monitoring of susceptibility patterns in <i>Pasteurellaceae</i> isolates from livestock.
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