الفهرس 
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Abstract
Inflammation of the mammary gland or mastitis is a serious problem for the dairy industry around the world, because of huge economic losses incurred due to poor quality and quantity of milk. It is caused by Staphylococcus aureus that is considered a major contagious pathogen, spreads rapidly in dairy herd during milking and leads to clinical and subclinical forms of mastitis.
The present study was done on 100 milk samples (60 cattle and 40 buffalo) randomly collected from different localities at Minufiya governorate to study the molecular characterization of Staphylococcus aureus isolated from these animals. Among the examined milk samples 70% (70/100) were belonged to subclinical mastitis, 83.33% (50/60) were from cattle samples and 50% (20/40) were from buffalo samples. However, 30% (30/100) were belonged to clinical mastitis, 16.67% (10/60) and 50% (20/40) of the examined cattle and buffalo milk samples, respectively.
For isolation and identification of S. aureus, the milk samples were cultured on specific media and among the examined 100 milk samples, 80% (80/100) gave S. aureus colonies. from the 70 subclinical cases, 92.85% (65/70) were grown on specific media, 90% (45/50) and 100% (20/20) of the cattle and buffalo’s milk samples, respectively. On the other hand, 50% (15/30) of the examined clinical cases were grown on specific media, 50% (5/10) and 50% (10/20) of the cattle and buffaloes milk samples, respectively.
For biochemical identification, the samples were tested for the coagulase activity and 27/80 (33.75%) from the isolates were coagulase positive S. aureus, 33.8% (22/65) from subclinical cases of mastitis and 33.3% (15/30) from clinical cases. However, 53/80 (66.25%) were coagulase negative Staphylococci.
After that, 10 isolates from 27 and 10 isolates from 53 coagulase positive S. aureus and coagulase negative Staphylococci isolates, respectively were randomly selected and tested for their antibiotics sensitivity patterns using (12) commercial antibiotics discs by antibiotic diffusion method. All the isolates 20/20 (100%) isolates were resistant (100%) to (10) commercial antibiotics (Oxacillin, Amoxicillin, Penicillin, Cefotaxime, Cephradine, Lincomycin, Ampicillin, Ciprofloxacin, Cloxacillin and Trimethoprim\ Sulphamethoxazole). In addition, 15/20 (75%) and 7/20 (35%) of the tested isolates were resistant to Norfloxacin and Gentamycin, respectively.
For further identification and comparison between the phenotypic and genotypic characterization of Staphylococci isolates, the 20 isolates (10 coagulase-positive S. aureus and 10 coagulase-negative Staphylococci) were subjected to genotypic characterization using PCR 16S rRNA, nuc and mecA genes. All the (20) isolates were positive (100%) for the 16S rRNA and nuc genes and only (9/20) (45%) were positive for mecA gene. Then, these isolates tested for the presence of virulence factor genes using PCR targeting Coa, tst and clfA genes. The result showed that all the (20) isolates were positive (100%) for Coa and clfA genes and only 2/20 (10%) isolates were positive for tst gene. Also, these isolates characterized using PCR targeting mecA, blaZ, Sul1, norA and aac (6’ ) aph (2’’) genes. The result showed that 9/20 (45%), 11/20 (55%), 20/20 (100%), 3/20 (15%) and 3/20 (15%) were positive for mecA, blaZ, Sul1, norA and aac(6’)aph (2’’), respectively.