الفهرس 
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Abstract
Ten selected strains of S. Typhimurium ( 5 chicken, 4 duck and one from turkey origin) isolated from poultry flocks in 5 geographical areas were characterized by phenotypic and genotypic methods. Non of the examined S. Typhimurium isolates were positive for infant mice assay. Meanwhile all of the examined isolates were positive for Verotoxin activity. All S. Typhimurium strains were pathogenic for one day old chicks with a total mortality of 56%, The antibiotic sensitivity test revealed that 100%, 80%, 70% and 70% of the isolates were sensitive to chloramphenicol, ciprofloxacin , enrofloxacin, and norfloxacin respectively. While 100% of the isolates were resistant to amoxicillin and ampicillin, and 70% were resistant to neomycin. S. Typhimurium showed a degree of variation in plasmid number (from 1 plasmid up to 6 plasmids) and molecular weight (from 583 b.p up to 16732 b.p). All S. Typhimurium strains harbored plasmid of 11425 b.p, while 40 % of the isolates had plasmid of 1893 b.p. Meanwhile plasmids of 16732, 1326 and 583 b.p were detected in 30% of the isolates. The largest plasmid of 16732 b.p was detected in 30% of our isolates that showed the highest incidence of antimicrobial resistance and mortality rates among chicks which may probably contributes towards high virulence of these isolates Restriction endonuclease analysis (REA) of the genomic DNA revealed that 24-34 bands were identified among isolates. 10 bands were common to all investigated strains.