Abstract
The spread of antibiotic-resistant pathogenic Escherichia coli poses a serious threat to calf health on livestock farms. With the decline in antibiotic therapy effectiveness, alternative approaches such as phage therapy are urgently needed. This study aimed to isolate lytic E. coli bacteriophages, characterize their properties, and evaluate the synergistic effects of their combined use with veterinary antibiotics against colibacillosis pathogens in calves. As a result of the work, 4 bacteriophages were isolated from wastewater from various cities of Kazakhstan: vB_EcoS_ABO/4, vB_EcoM_PL/4, vB_Eco_CWW/26, vB_EcoM_ShWW/46. Morphological, biological, and genomic analyses showed that the phages belong to different genera of the Caudoviricetes class, possess high lytic activity, broad host range, environmental stability, and lack genes associated with lysogeny, antibiotic resistance, or virulence. Interaction studies with antibiotics revealed synergistic or additive effects in over 75% of cases. These findings highlight the strong potential of the isolated bacteriophages for independent or adjunctive use in the treatment and prevention of colibacillosis in calves. However, further in vivo studies are required to definitively confirm their therapeutic efficacy.