الفهرس 
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Abstract
Mastitis is considered one of the most widespread and costly infectious diseases affecting the dairy industry. S. aureus is one of the most common causes of both clinical and subclinical bovine mastitis, which greatly affects the Egyptian dairy industry by decreasing milk production, increasing milk contamination, milk disposal, and treatment costs. Due to the irrational usage of antibiotics, the bacteria has developed many resistance genes of bovine-originated strains. These strains can be transported into the human which represents a medical threat to the public health.
This study was performed over 200 milk samples isolated from clinical and subclinical mastitic case. S. aureus prevalence rate was 23% (46/200, 42 samples were isolated from clinical cases and 4 samples were isolated from sub-clinical cases). S. aureus strains were confirmed using PCR for amplification of nuc gene, (13.04%) of S. aureus isolates were identified as MRSA (mecA positive).
MSSA isolates showed an absolute resistance to ampicillin (100%; 40/40), followed by penicillin and spiramycin (92.5%; 37/46 each), streptomycin(80%;32/40), Tetracycline (77.5%;31/40), , erythromycin (72.5%;29/40), Vancomycin (67.5%;27/40) and Gentamycin (65%;26/40) , ciprofloxacin(30%;12/40), sulfamethoxazole/trimethoprim (25%;10/40) and lower resistance to marbofloxacin(7.5%;3/40). While showed susceptibility to oxacillin (100%; 40/40),Ceftifuor (100% ;40/40) imipenem (100%;40/40) amoxicillin-clavulanic acid , ceftriaxone (90%; 36/40), cefotaxime and cefuroxime (75%;30/40 each). While showed susceptibility to oxacillin (100%; 40/40),Ceftifuor (100% ;40/40) imipenem (100%;40/40), ceftriaxone (90%; 36/40), amoxicillin-clavulanic acid ,cefotaxime and cefuroxime (75%;30/40 each). Meanwhile, MRSA isolates showed an absolute resistance to ampicillin (100%; 6/6), oxacillin (100%; 6/6),Ceftifuor (100% ;6/6) , penicillin (100%; 6/6),spiramycin (100%;6/6), ceftriaxone,amoxicillin-clavulanic acid ,cefotaxime and cefuroxime, erythromycin (83.3%;5/6 each), streptomycin,Vancomycin , Gentamycin ,Tetracycline(66.66%; 4/6 each) , While showed susceptibility to imipenem (100%;6/6), sulfamethoxazole/trimethoprim (83.3%;5/6) and marbofloxacin (50%;3/6). All S. aureus isolates were MDR, and the most identified antimicrobial resistance patterns was: S,SP,P,AM,CN,TE,E,VA,SXT . Furthermore, antibiotypes identified in this study had MAR index values ranged from 0.22 to 0.77 Silver nanoparticles (Ag-NPs) with a concentration 50 ug/ml showed antibacterial effects against both MSSA and MRSA strains.Silver nanoparticle (2ug/ml) /antibiotics (Ag-NPs/AB) combinations have shown higher antimicrobial activity than free antibiotics only. MSSA were sensitive to Ag-NPs(2ug/ml) / Tetracycline (30ug) combination (92.5%), Ag-NPs (2ug/ml) / Spiramycin (100ug) combination (100%) and Ag-NPs(2ug/ml) / Amoxycillin-Clavulanic Acid (30ug) combination (100%). Also, MRSA isolates were sensitive to Ag-NPs(2ug/ml) / Tetracycline (30ug) combination by (100%), Ag-NPs (2ug/ml) / Spiramycin (100ug) combination by (100%) and Ag-NPs(2ug/ml) / Amoxycillin-Clavulanic Acid (30ug) combination by (100%). According to the current study silver-nanoparticles can be used in treatment of S. aureus mastitis either alone or combined with the commercial antibiotics.