الفهرس 
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Abstract
Escherichia coli is one of the most important bacteria affecting poultry and causes diseases such as Colibacillosis which is the main cause of morbidity and mortality in poultry and is responsible for great economic losses worldwide. This disease may be a primary disease or secondary to other diseases.
The total number of samples was 85 isolates collected liver, heart, kidney, and oviduct from broilers from the El-Behera governorate. Chickens were suffering from pericarditis, perihepatitis, omphalitis. The collected samples were immersed in nutrient broth.
The collected samples were immersed in the nutrient broth and incubated at 37oC for 18-24 hours. A loopful from each sample was cultured onto MacConkey’s agar to obtain pure pink colonies due to lactose fermentation. Some biochemical tests used for biochemical identification of E. coli as an indole test, Methyl red test, Citrate utilization test, Urease test, and Catalase test. By biochemical testing, the biochemically positive E. coli was 66 isolates with a percentage 77.6%.The isolates were tested for hemolytic activity using 5% sheep blood agar and showed 80.3% of the isolates gave alpha hemolysis and 19.7% give beta hemolysis.
The isolates were examined for their sensitivity to ten antibiotics. out of 66 E. coli isolates were tested for antibacterial sensitivity, most of E. Coli resist Ampicillin and Amoxicillin with percentage 59.09% and 56.06%, respectively, moderately sensitive to Nitrofurantoin, Neomycin, Tetracycline, Gentamycin, Cephalexin, Kanamycin and Chloramphenicol with percentage 87.87%, 83.33%, 83.33%, 74.25%, 72.72%, 59.09% and 57.57% respectively and sensitive to Ciprofloxacin with percentage 53.03%. Out of 66 biochemically positive E. coli isolates, 21 isolates (31.8%) were positive for the phoA gene. PCR also used for the detection of five virulence genes. Out of 21 PCR positive E. coli., only 9 isolates (42.85%), 2 isolates (9.53%), 10 isolates (47.6%), 12 isolates (57.14%) and 7 isolates (33.33%) were positive for papC, iutA, fimH, iss and fyuA gene, respectively. from those results, there were many virulence factors affecting pathogenicity of E. coli as papC, iutA, fimH, iss, fyuA gene and the most predominant one was increased serum survival gene (iss) followed by fimH gene and papC c gene which responsible for initial colonization and adhesion of bacteria. Antibiotic selectivity test is the best method to choose the most effective antibiotic and avoid drug resistance. In this study, the most effective antibiotic was Ciprofloxacin that was considered the drug of choice and avoid using ampicillin and amoxicillin because of the high resistance of bacteria against those antibiotics.