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.

The search for novel antiviral agents against Newcastle disease virus (NDV) remains a priority in industrial poultry farming due to the virus’s high contagiousness and associated economic losses, prompting evaluation of polyene macrolides as potential therapeutic candidates. We employed a comprehensive approach combining computational modeling (molecular docking and dynamics simulation) and laboratory experiments to investigate the antiviral potential of natamycin, nystatin, and filipin complex against three NDV strains. Molecular docking analysis indicated binding sites for macrolides within the hydrophobic regions of surface glycoproteins HN and F, with binding energies ranging from −6.5 to −10.5 kcal/mol, while 50 ns molecular dynamics simulation confirmed complex stability. Laboratory testing using fluorescence-based neuraminidase assays demonstrated dose-dependent inhibitory activity with IC₅₀ values of 0.0043 ± 0.0015 mg/mL for filipin complex, 0.0117 ± 0.0029 mg/mL for nystatin, and 0.0220 ± 0.0138 mg/mL for natamycin, with similar ranking observed for fusion inhibition (EC₅₀ values of 0.00053 ± 0.00039, 0.00545 ± 0.00560, and 0.01196 ± 0.00965 mg/mL, respectively). While filipin complex exhibited the highest antiviral activity, its significant cytotoxicity limits therapeutic application, whereas natamycin demonstrated a favorable safety profile consistent with its GRAS status. These findings indicate that natamycin exhibits a favorable safety-to-efficacy profile in vitro, warranting further in vivo investigation to clarify its mechanism of action and establish practical application protocols for NDV control in poultry.