الفهرس 
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Abstract
Escherichia coli bacterium is common to many environments .The introduction of such strains to chicken respiratory tract causes invasive infections, collectively known as collibacillosis. It can cause extensive mortality in poultry flocks leading to great economic losses. Recent reports showed that the APEC pathogenicity is associated with some virulence genes which located within the bacterial genome. The study aimed for molecular characterization of virulence associated genes of Escherichia coli in broilers suffering from coli septicemia (APEC). Five steps have been done for achieving the aim including, isolation and identification of E. coli from broilers suffering from coli septicemia from 10 different broiler farms at Behira governorate followed by Serotyping of these E. coli isolates then characterize the pathogenicity of them and testing their susceptibility to 12 antimicrobial agents and finally detection of five virulence associated genes in the E. coli isolates via polymerase chain reaction. The prevalence study of E. coli was carried out on ten broiler farms in Behera Governorate. A total of 300 samples of broilers suspected collibacillosis collected for isolation and identification of APEC. The samples had been taken from internal organs and also cloaccal swabs of diseased chickens. 185 samples were positive for E. coli. For detection the APEC, many biochemical tests have been carried out and serological detection of the eleven E. coli isolates by Polyvalent and Monovalent serum in Mast Diagnostic kit, then experimental infection on lab animals (white mice) have been done and found that 32.4% of the E. coli strains were pathogenic then Antimicrobial susceptibility testing were carried out for testing their Susceptibility to twelve antimicrobial agents. The results showed a high resistance to the Tetracycline30/ μg (TE) was (90%), Nalidixic acid 30/ μg (NA) (100%) , Amoxicicilline \Clavulinicacid (Amc) (20\10) / μg was (100%), Ofloxacin 5/ μg (OFX)was (70%), and Erythromycin15/ μg (E) was (80%) ,while on the other hand all the isolates showed high susceptibility to Nitrofurantoin 300/ μg (F)and cefotaxime 30/ μg (CTX)with percentage of (100%).
Finally, the Nine serotypes of E. coli isolates examined for detection of the 5 virulence genes (iss, iutA, papC, tsh and TraT) using PCR. And their percentage were Iss (100%), TraT (100%), papc (88.9%),iutA (100%)and tsh(33.3%).