الفهرس 
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Abstract
The objectives of the study were to check the occurrence of Methicillin-Resistant Staphylococcus aureus (MRSA) and the molecular characteristics of the isolates within the nasal carriage of healthy human, cattle and sheep. Alpha-hemolysin (Hla) is a major virulence factor in the pathogenesis of Staphylococcus aureus infection, being active against a wide range of host cells. We evaluated nucleotide sequence variation and gene expression profiles of hla among representatives of Methicillin-Resistant Staphylococcus aureus (MRSA) and Methicillin-Susceptible Staphylococcus aureus MSSA from the nasal carriage of apparently healthy sheep and human.One hundred and fifty animal samples (100 from sheep, 50 from cattle) and 100 samples from humans in contact with animals were collected in the study. Human and animal nasal swabs samples were collected at Basateen slaughter house. The isolation and identification of S. aureus isolates were applied according to traditional biochemical tests. Disc diffusion test was performed to record resistance and MRSA isolates. PCR was done for detection of nuc, and mecA genes. A total of twenty-three isolates from which 3 MRSA isolated from sheep (n=1) and human (n=2), while twenty MSSA isolates from sheep (n=2) and human (n=18). Were assessed phenotypically for the ability of hemolysis either qualitatively or quantitively. Multiplex PCR was carried out for the detection of genotypic hemolytic pattern, hla genewas sequenced for phylogeny and gene expression was assessed by RT-PCR. Only 60 out of 250 nasal swab samples produced S. aureus (24%), and 10 of which were MRSA (16.6%). All S. aureus isolates were sensitive to vancomycin and resistant to oxacillin, cefoxitin and erythromycin. Two isolates from sheep nasal swabs were intermediate resistant to cefoxitin and ofloxacin (50%). All isolates were nuc gene positive, while two out of the 10 MRSA isolates (20 %) were mecA negative, whereas all the methicillin sensitive S. aureus (MSSA) were mecA negative. All the S. aureus isolated from sheep showed beta hemolysin activity and only one ovine strain showed alpha hemolysis, while all the human isolates produced alpha hemolysin Out of 23 strains tested 21 strains were positive for alpha hemolysin gene, we recorded absence of the hla gene in two ovine strains and its presence in the only ovine MRSA isolate. Most common gene profile in human isolates was hla, hld hemolysin associated genes which was harbored by 14 (70%) isolates, while beta gene was detected in all human MRSA isolates and 4 out of 18 human MSSA isolates .Therefore S. aureus and MRSA from sheep, cattle and human are considered a potential risk for zoonotic transmission, this study drew attention to the credibility of the mecA gene and its usefulness in the detection of all MRSA strains without referring to the traditional methods.Phylogenetic analysis of hla gene, suggest that zoonotic transmission. hla expression level was high in all MRSA, while all the MSSA isolates were in the low expression group which make it a potential target for new anti-virulence therapeutics and a candidate for vaccine studies.