Summary: | Mastitis is an infectious disease encountered in dairy animals worldwide that is currently a growing concern in Lebanon. This study aimed at investigating the etiology of the main mastitis-causing pathogens in Northern Lebanon, determining their antimicrobial susceptibility profiles, and identifying their antimicrobial resistance (AMR) genes. A total of 101 quarter milk samples were collected from 77 cows and 11 goats presenting symptoms of mastitis on 45 dairy farms. Bacterial identification was carried out through matrix-assisted laser desorption/ionization-time of flight mass spectrometry. Antimicrobial susceptibility was tested by disc diffusion and broth microdilution methods. Molecular characterization included polymerase chain reaction (PCR) screening for genes encoding extended-spectrum beta-lactamases (ESBLs) and plasmid-mediated <i>AmpC</i> among <i>Enterobacterales</i> isolates, and virulence factors among <i>Staphylococcus</i> isolates. <i>Escherichia coli</i> isolates were subjected to phylogenetic typing by a quadruplex PCR method. The most frequently identified species were <i>Streptococcus uberis</i> (19.2%), <i>Streptococcus agalactiae</i> (15.1%), <i>E. coli</i> (12.3%), and <i>Staphylococcus aureus</i> (10.96%). Gram-positive bacteria were resistant to macrolides and tetracycline, whereas gram-negative bacteria displayed resistance to ampicillin and tetracycline. Two ESBL genes, <i>bla</i><sub>TEM</sub> (83.3%) and <i>bla</i><sub>OXA</sub> (16.7%), and one <i>AmpC</i> beta-lactamase gene, <i>bla</i><sub>CMY-II</sub> (16.7%), were detected among six <i>E. coli</i> isolates, which mainly belonged to phylogenetic group B1. Among <i>Staphylococcus</i> spp., the <i>mecA</i> gene was present in three isolates. Furthermore, four isolates contained at least one toxin gene, and all <i>S. aureus</i> isolates carried the <i>ica</i> operon. These findings revealed the alarming risk of AMR in the Lebanese dairy chain and the importance of monitoring antimicrobial usage.
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