In this study, 90 internal organ samples of poultry with symptoms of colibacillosis were obtained from Maragheh poultry farms in East Azerbaijan, Iran. In total, 70 bacterial isolates were confirmed as Escherichia coli (E. coli) strains using standard biochemical tests, and antibiotic sensitivity was determined by the disk diffusion method. Antibiotics used in this study included ampicillin, penicillin, nitrofurantoin, tetracycline, amoxicillin, ciprofloxacin, nalidixic acid, and sulfamethoxazole (n=8). Ciprofloxacin showed the highest susceptibility, while the lowest susceptibility was observed with penicillin and amoxicillin. Among the bacterial isolates, 50% showed resistance to at least five antibiotics, and 10 isolates with multidrug resistance were selected for bacteriophage (phage) isolation against recent E. coli isolates using spot test and double-layer agar overlay technique. In addition, water samples for phage isolation were provided from rivers, poultry farm sewages, and an urban sewage treatment center. In total, eight phages were successfully isolated from the urban sewage treatment center (total: 10). After enrichment, purification and titration, phages were further concentrated by polyethylene glycol precipitation. Lowest and highest bacteriophage titers were determined to be 1.05×106 and 1.9×109 PFU/ml, respectively. Host range of the isolated phages was assayed by spot testing, and antibacterial effects against four E. coli isolates were observed in one of the isolated phage suspensions, which was introduced as the most potentiated agent for phage therapy. In the morphological analysis of the selected phage using an electron microscope, we observed a hexagonal head with a diameter of 95 nm and contractile tail length of 90 nm, which indicated its similarity to the Myoviridae family. In conclusion, results of this study showed that bacteriophages could be appropriate alternatives to combat pathogenic E. coli strains with antibiotic resistance in po
