الفهرس 
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Abstract
Escherichia coli infections are among the most significant infectious diseases that affect poultry industry causing high economic losses not only due to high mortality, production losses, condemnations but also for the debilitating effect which predisposing for many other diseases. Therefore, this study was designed to throw spot lights upon E. coli isolated from broilers, studying their cultural, biochemical, serological characters and In-vitro anti-microbial Sensitivity for them with special reference tosome virulence genes. So, the present study was performed on a total of 44 commercial broiler farms (20- 30 days old) from four different areas (Benha; kaferShoker; Tokh and Shebin El- Kanater); 11 farm from each one; at Kaliobia Governorate were inspected for E.coli infection. Samples were taken from 68 diseased chicken and 152 freshly dead ones (liver; heart blood; lung; intestine; kidney and spleen from each chicken) after clinical and postmortem examination. The results of E.coli isolation revealed that 502 out of 1320 samples were positive for E.coli isolation, where 124 strains (24.7%) isolated from 408 samples of 68 diseased chicken and 378 strains (75.7%) from 912 samples of freshly dead chicken.Moreover, higher rates of isolation of E.coli from intestine (31.3%); liver (28.1%); heart blood (16.1%); spleen (9. 6%); kidneys (8.0%) and finally lungs (7.0%). Regarding the serological identification of randame (15) E.coli isolates, 11 (73.3%) were polyvalent antisera (2) while other 4 (26.7%) untyped. Also,only three serogroups of E. coli (O55, O125 and recently O146) were identified serologically represented as 6 strains were serotype O146 (40%); 3 O125 (20%); 2O55 (13.3%) and 4 isolates were untyped by the available antisera. Moreover, the serogroups O55 and O146 were positive for K99 (virulence factor) while O125 was negative. The results of antibiotic sensitivity tests, revealed that Gentamycin; Cefotaxime; Ampicillin/ Clavulinic acid and Enrofloxacin were the most proper antibiotics with the highest in vitro efficiency against isolated E.coli.Meanwhile, all the E.coli isolates showed weak sensitivity for Neomycin, Erythromycin and Trimethoprim/ Sulphamethoxazol and very high resistance to Amoxicillin ,Pefloxacin, Streptomycin and Oxytetracycline .This is of serious concern because these drugs are still considered the most recommended for the treatment of colibacillosis in both animal and human .Multiplex PCR results showed that eaeA; ompA; kpsMTII; tsh; iutA and iss virulence genes were detected in all serogroups. While papC virulence gene was detected in serotype O55 only. Moreover, stx2 virulence gene was detected in both serotypes O55 and O125. Meanwhile, it was not amplified in O146 serotype. The present study clearly demonstrates that E. coli is one of the major serious pathogen responsible for various types of disease conditions in broilers resulting in high mortality and morbidity, as they are isolated with high percentages and multiplex PCR indicated that, all serotypes isolated (O55, O125 and recently O146) had the eaeA; ompA; kpsMTII; tsh; iutA and iss virulence genes; Meanwhile papC virulence gene was detected in serotype O55 only and stx2 virulence gene was detected in both serotypes O55 and O125. Moreover, the serogroups O55 and O146 were positive for K99 (virulence factor) while O125 was negative, and all of these genes play a role in pathogenicity and virulence of APEC. Finally, Gentamycin; Cefotaxime; Ampicillin/ Clavulinic acid and Enrofloxacin were the most proper antibiotics with the highest in vitro efficiency against isolated E.coli Which can be used for treatment in cases of E.coli infections.