Isolation, Identification, and In Vivo Anti-Infective Efficacy of Lytic Bacteriophage SEP1 Against Salmonella Paratyphi

Background The emergence of multidrug-resistant Salmonella Paratyphi has created a critical need for novel therapeutic options beyond conventional antibiotics. Bacteriophages, with their ability to selectively lyse bacterial hosts, offer a promising alternative. This study aimed to characterize the lytic phage SEP1 and evaluate its efficacy against S. Paratyphi infection. Methods SEP1 was isolated from poultry farm sewage in Lanzhou, China, using S. Paratyphi QH as the host. We performed transmission electron microscopy for morphological analysis, double-layer agar assays for host range, one-step growth curves for lifecycle characteristics, and stability tests under varying temperature and pH conditions. Genomic DNA was sequenced and analyzed. In vivo efficacy was assessed in a lethal murine model of S. Paratyphi infection, with monitoring of survival, body weight, bacterial loads in liver and spleen, histopathology of liver/spleen/intestine, and serum cytokine levels (IL-1β, IL-6, IL-10, IFN-γ). Results SEP1 was classified as a member of the Felixounavirus genus, featuring a double-stranded DNA genome of 85,703 bp with 39.00% GC content and 123 predicted open reading frames. No lysogenic or virulence genes were identified. The phage exhibited a host range infecting 31.25% of tested Salmonella strains, a latent period of 20 minutes, and a burst size of approximately 141 PFU per cell. It remained stable at temperatures between 10–50°C and pH values from 4 to 9. In the mice model, a single dose of SEP1 administered 30 minutes post-infection resulted in 100% survival, compared to 0% in the challenge control group (p < 0.001). Treatment prevented weight loss, reduced bacterial loads in organs (p < 0.01), mitigated tissue damage, and restored cytokine levels to near baseline.