الفهرس 
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Abstract
Three hundred and fifty quarter milk samples were collected from apparently healthy cows at Al-Gharbia governorate, Egypt. Samples were tested using California Mastitis Test (CMT) and somatic cell count, of which 235 samples were detected as positive. Overall, 168 quarter milk samples (71.48%) were found to be contaminated with coagulase positive Staphylococcus aureus with a mean value of 64x103± 19x103cfu/ml. Isolated Staphylococcus aureus strains were tested for methicillin to identify Methicillin resistant strains (MRSA). Antibiotic sensitivity test was carried out by using two antibiotic disks against fourty six identified Staphylococcus aureus isolates. The obtained results indicated that resistance against cefoxitin was 34.78% and sensitivity was 65.22%, while the resistance against vancomycin was 26.08% and sensitivity was 73.92%. PCR technique was used to detect presence of mecA gene that coded for penicillin-binding protein 2a. The results on sixteen positive isolates which suspected to have mecA gene by antibiotic sensitivity test were 93.75%. The total of subclinical mastitis cases infected with MRSA was 31.11%. The results provided evidence that the presence of Staphylococcus aureus, as well as Methicillin–resistant strains (MRSA) have become remarkably widespread in subclinical mastitic quarter milk samples. This calls for better control of the sources of milk contamination as well as spread of antimicrobial resistance organisms.