Prevalence and molecular characterization of resistant <i>Staphylococcus aureus</i> strains in bulk milk tanks of dairy cattle in Northern Egypt.

Bovine subclinical mastitis represents a major cause of severe economic losses in dairy farms. This research aimed to detect the antimicrobial resistance trends of <i>Staphylococcus aureus</i> and to determine the presence of <i>mecA,</i> <i>mphC, lnuA, tetK</i> and <i>tetL</i> antimicrobial resistance genes in raw bulk milk in the period between December 2023 and February 2024. One hundred raw bulk cow milk samples were gathered from different dairy farms in Egypt. The prevalence of subclinical bovine mastitis was 65.00% using California mastitis test. The prevalence of isolated <i>S. aureus</i> was 46.15% via bacterial culturing and all isolates (n = 30) were confirmed via hemolytic activity, catalase and coagulase test, and gram staining followed by polymerase chain reaction targeting <i>nuc1</i> gene. Antimicrobial sensitivity test was applied on all confirmed <i>S. aureus</i> isolates utilizing the disk diffusion method on Mueller-Hinton agar. The highest resistance was verified for tetracycline at 100% followed by erythromycin and clindamycin at 56.66 and 16.66%, respectively. The highest sensitivity at 100% was verified for amikacin, ampicillin, amoxicillin plus clavulanic acid, ampicillin plus sulbactam, ciprofloxacin, colistin, gentamicin, imipenem, tobramycin, doxycycline and vancomycin. Multidrug resistance was found in 20.00% of the total isolates. Methicillin resistant <i>S. aureus</i> represented by <i>mecA</i> gene was identified in 83.33% of isolates. Macrolides resistant <i>S. aureus</i> represented by <i>mphC</i> gene was identified in 16.66% of isolates. Lincosamide resistant <i>S. aureus</i> represented by <i>inuA</i> gene was identified in 66.66% of isolates. Tetracycline resistant <i>S. aureus</i> represented by <i>tetK</i> and <i>tetL</i> genes was detected in 23.33 and 53.33% of isolates, respectively. This study provided antibiotic-resistant <i>S. aureus</i> profiles to dairy farms to avoid treatment failure, adverse effects on animal health and economic impact for the owner of the animal.