الفهرس 
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Abstract
The aim of this thesis was to investigate the antimicrobial activity of vancomycin, teicoplanin, gentamicin and rifampicin, each alone and in combinations, against selected Gram-positive clinical isolates collected from different sources in Egypt. It also aimed to study the distribution of the genes conferring resistance to these antibiotics among some of the selected isolates.
A total number of 86 bacterial clinical isolates obtained from different sources were used in this study, in addition to the two standard strains: Staphylococcus aureus ATCC 6538p and Staphylococcus epidermidis ATCC 12228. The isolates were identified by classical microscopical and biochemical methods as follows: 49 S. aureus, 14 S. epidermidis, 20 E. faecium and 3 E. faecalis. The identified clinical isolates were tested for their susceptibility to different antibiotics using the disc diffusion technique. Out of the eighty-six collected clinical isolates: 58 were resistant to CN, 34 were resistant to RA, 4 were resistant to TEC and only 3 isolates were resistant to VA.
The antimicrobial activity of each of VA, TEC, CN and RA against the collected clinical isolates was investigated through MIC determination using the agar dilution method. Among the staphylococcal clinical isolates, the MIC ranges lied between (0.25-4 μg/ml) for VA, (0.5-16 μg/ml) for TEC, (0.125- >1024μg/ml) for CN and (0.016- >1024μg/ml) for RA. For the enterococcal clinical isolates, the MIC ranges lied between (0.5-512 μg/ml) for VA, (0.25-256 μg/ml) for TEC, (0.5- >1024 μg/ml) for CN and (0.063-128 μg/ml) for RA.
On the other hand, the broth dilution method was used to determine the MICs of VA, TEC, CN and RA for selected isolates as a prerequisite for the checkerboard technique and the time-kill assay. It was noticed that there was almost no considerable difference between the agar and broth dilution methods, compared to the results obtained by the antibiotic disc diffusion method except for the isolates possessing marginal sensitivity or intermediate levels of resistance to VA and/or TEC. For further confirmation, the standard E-test was carried out for selected isolates and was compared to the three previously mentioned methods.
from such comparison, it was concluded that the antibiotic disc susceptibility method could only be helpful in case of the detection of complete resistance to glycopeptides, but it was considered to be of low sensitivity in the detection of intermediate levels of resistance to these antibiotics. On the other hand, both the agar and broth dilution methods were considered to be the most sensitive and reliable methods for the detection of complete and intermediate resistance of Gram-positive cocci to glycopeptides. Concerning the E-test, our results showed that this method was characterized by a high specificity (no false positive results were detected), but it was of somewhat low sensitivity (due to some false negative results). Therefore, it could result in the underestimation of intermediate or complete resistance to glycopeptides. Moreover, its high cost might render its use inapplicable on large scales.