Characterization and therapeutic evaluation of a novel phage S18-3 targeting poultry-derived Enterobacter hormaechei infection in mice

BACKGROUND: Enterobacter hormaechei (E. hormaechei) is an opportunistic pathogen in poultry, increasingly associated with multidrug resistance due to the extensive use of antibiotics in animal production. The emergence of resistant E. hormaechei strains poses a significant threat to poultry health and food safety, highlighting the urgent need for alternative antimicrobial strategies. This study aimed to isolate and characterize a novel lytic bacteriophage targeting E. hormaechei and to evaluate its biological properties and therapeutic potential in a gastrointestinal infection mice model. RESULTS: A lytic phage, designated S18-3, was isolated from environmental water using E. hormaechei strain HZW1 isolated from diseased broiler liver. Phage S18-3 forms clear plaques (~ 0.6 cm) and exhibits an icosahedral head with a long tail. Optimal infectivity occurs at an MOI of 0.1. S18-3 remains stable from 30 to 50 °C and tolerates a broad pH range (pH 4-11). Whole-genome sequencing revealed no known virulence factors or antimicrobial resistance genes, indicating favorable genetic safety. In a murine intestinal infection model established by antibiotic pretreatment and oral challenge, E. hormaechei caused significant weight loss and mortality. Oral administration of S18-3 markedly improved survival and mitigated weight loss compared with infected controls. Phage S18-3 titers were detectable in feces in infection group, with only transient, low-level presence in blood. Histopathology showed no evident liver injury or inflammatory changes, suggesting no hepatotoxicity associated with phage treatment. CONCLUSIONS: Phage S18-3 is a stable, genetically safe lytic phage that effectively alleviates intestinal E. hormaechei infection in a murine model without observable adverse effects. These data support the potential of S18-3 as an antibiotic alternative for controlling E. hormaechei infections in poultry. Further validation in broiler models and farm-level settings is warranted to assess translational applicability in sustainable poultry production.