الفهرس 
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Abstract
In the current study bacteriological examination of 100 samples collected from broiler chicken’s internal organs revealed that, the overall isolation rate of E. coli from the internal organs of broiler chickens with signs of colibacillosis was 90% (90/100). Regarding the site of isolation, E. coli isolates were recovered from lung, liver, heart blood and kidneys with prevalence of (94.1%), (96.7%), (85.7) and (73.3%) respectively.
Serogrouping of 10 randomly selected E. coli isolates revealed that the most prevalent serogroups isolated were O158 and O27 with the prevalence rate of 20% for each. A significant percentage of the isolates APEC (20%) were untyped.
Antibiogram of 30 E. coli isolates were evaluated, and the result confirmed that all the tested isolates (100%) showed multidrug resistance (MDR) pattern. The resistance pattern was recorded as following, Amoxicillin-clavulanic acid (AMC), Streptomycin (S), Spectinomycin (SH) and Tobramycin (100% for each) followed by Doxycycline hydrochloride (DO) and Kanamycin (K) (93.3% and 86.6%) respectively. Come after that Apramycin (APR), Ceftriaxone (CTR) and Colistin sulphonate (CL) (76.7% for each). E. coli isolates also showed varying degrees of resistance towards ciprofloxacin (CIP) and Gentamicin (GN) (70% and 63.3%) respectively, aztreonam (ATM) and Co-trimoxazole (COT) (60% for each) and Amikacin (Ak) (56.7%). In contrast high sensitivity was recorded for Fosfomycin (FO) (76.7%)
screening of seven virulence genes in 5 serogrouped MDR E. coli isolates using PCR revealed that the targeted virulence genes (fimH, fimA, iutA, and papC) were found in 100% of the isolates, whereas (fim AVMT78 and tsh) genes were only discovered once in the APEC O78 and O111 serogroups respectively, with a prevalence of 20% for each. However, none of the examined isolates contained the felA gene.
In the experimental design, chickens challenged with E. coli O78 by oral or intraperitoneal injection showed significant clinical signs in comparison to control negative group with mortality rate (5%) in each of the two challenged groups.
The immune status of the chickens under experiment and naturally infected chickens were evaluated in comparison to non-infected chickens. and the results revealed elevated levels of (lysozymes, NO, CD4 receptor protein and IL1β) after infection by E. coli either naturally or experimentally.
The HI test was applied to monitor the humoral immune response of vaccines against the antigens of NDV and avian influenza H9N2 and H5N1viruses in the infected and non-infected groups and the results revealed that the humoral immune responses to the three-vaccines decreased to some extent with all experimentally vaccinated and E. coli infected groups compared to higher HI titers in vaccinated healthy chickens.