الفهرس 
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Abstract
One of diseases challenging veterinarians and producers nowadays is colibacillosis that leads to considerable economic loss to producers and transmission of resistance genes of antimicrobial among bacterial isolates is an increasing problem.
Therefore the objective of this study was to characterize APEC strains causing respiratory troubles in chickens. In the present work, 30 broiler chicken flocks from Sharkia, Ismilia, Dakahlia and Sinai were tested. The clinical and postmortem examination of ninety birds revealed typical findings of colisepticemia including respiratory troubles, greenish diarrhea, fibrinous pericarditis, fibrinous perihepatitis, air sacculitis and enteritis. 284 samples were collected from broiler Flocks at age of 17-35 days from different localities, which revealed high isolation rate of E.coli 193 (67.96%) isolates. The higher percentage of E.coli isolation was from Air sacs (76.1%) while the lowest rate from the liver (54.9%).
Isolation of E.coli isolates was on MacConkey agar medium and EMB agar medium. E.coli identification was confirmed by Catalase test positive (white foam), Citrate test negative (remain green), Indol test positive (red ring), urease test negative (yellow color).
Serotyping of E.coli isolates showed 12 serotypes. The most prevalent serotype O78, O2 with percentage of (15%) for each one then O44, O128, O158 (10%) for each one, O145.