الفهرس 
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Abstract
The aim of the present thesis is to detect some genes responsible for β-lactamase production and methicillin resistance and to compare between phenotypic and genotypic testing of methicillin resistance and β-Lactamase production by Staphylococcus aureus (S. aureus).
A total of fifty isolates of S. aureus were recovered from different clinical sources by isolation on Mannitol salt agar media and biochemically identified as S. aureus. The antimicrobial susceptibility pattern was estimated and the tested clinical isolates were phenotypically screened for β-Lactamase production. Twenty isolates were also genotypically examined by PCR for the presence of genes responsible for β-Lactamase production and methicillin resistance, blaZ and mecA genes.
Among fifty isolates of S. aureus, the antimicrobial susceptibility testing showed that all isolates were resistant to Penicillin and Ceftazidime. S. aureus isolates showed a relatively high resistance for Ampicillin, Amoxicillin/clavulinic acid, Cefipime and Ampicillin/sulbactam of 80%, 62%, 58% and 52% respectively. A relatively low resistance of isolates was recorded for Cefoperazone/sulbactam (14%), Amikacin (20%), Cefotaxime (24%), Pipercillin/tazobactam (26%), Oxacillin (32%), Cefoxitin (36%) and Ofloxacin (40%). 78% of isolates were intermediate resistant to Erythromycin and 62% were intermediate resistant to Cefoperazone. The most potent antimicrobial agent was Vancomycin, for which all isolates were susceptible.
Three isolates were susceptible to 12 out of the 16 antimicrobial agent tested and four isolates were susceptible to 11 out of the 16 antimicrobial agent tested. On the other hand, four isolates were resistant to all antimicrobial agent tested except Vancomycin and one isolate were resistant to all antimicrobial agent tested except Vancomycin and Eryhthromycin.
The phenotypic detection of β-lactamase using acidimetric test was carried and the percentage of β-lactamase producers among the isolates was found to be 94%. On the other hand, nitrocefin test for the detection of β-lactamase activity showed that 86% of isolates were β-lactamase producers.
For genotypic detection, it was found that 75% of the screened isolates (15 isolates) were positive for blaZ gene (all of them were β-lactamase producers), 70 % of the screened isolates (14 isolates) were positive for mecA gene (7 isolates were oxacillin resistant and 7 isolates were oxacillin susceptible).
The present study shows that acidimetric method is more sensitive for detecting staphylococcal β-lactamases. Vancomycin was found to be the best anti-staphylococcal drug in this study and cefoperazone/sulbactam was the most active combination used in this study. Also, this study show that genotypic characterization was more prominent compared to the phenotypic detection of the β-lactamase production and methicillin resistance. These findings confirm the importance of these genes in β-Lactamase production and methicillin resistance and conclude the presence of other factors which may contribute to β-Lactamase production and methicillin resistance in S. aureus.