Isolation and identification of the broad-spectrum high-efficiency phage vB_SalP_LDW16 and its therapeutic application in chickens

Background: Salmonella infection in livestock and poultry causes salmonellosis, and is mainly treated using antibiotics. However, the misuse use of antibiotics often triggers the emergence of multi-drug-resistant Salmonella strains. Currently, Salmonella phages is safe and effective against Salmonella, serving as the best drug of choice. This study involved 16 Salmonella bacteriophages separated and purified from the sewage and the feces of the broiler farm. A phage, vB_SalP_LDW16, was selected based on the phage host range test. The phage vB_SalP_LDW16 was characterized by the double-layer plate method and transmission electron microscopy. Furthermore, the clinical therapeutic effect of phage vB_SalP_LDW16 was verified by using the pathogenic Salmonella Enteritidis in the SPF chicken model. Results: The phage vB_SalP_LDW16 with a wide host range was identified to the family Siphoviridae and the order Caudoviridae, possess a double-stranded DNA and can lyse 88% (22/25) of Salmonella strains stored in the laboratory. Analysis of the biological characteristics, in addition, revealed the optimal multiplicity of infection (MOI) of vB_SalP_LDW16 to be 0.01 and the phage titer to be up to 3 x 10(14) PFU/mL. Meanwhile, the phage vB_SalP_LDW16 was found to have some temperature tolerance, while the titer decreases rapidly above 60 degrees C, and a wide pH (i.e., 5-12) range as well as relative stability in pH tolerance. The latent period of phage was 10 min, the burst period was 60 min, and the burst size was 110 PFU/cell. Furthermore, gastric juice was also found to highly influence the activity of the phage. The clinical treatment experiments showed that phage vB_SalP_LDW16 was able to significantly reduce the bacterial load in the blood through phage treatment, thereby improving the pathological changes in the intestinal, liver, and heart damage, and promoting the growth and development of the chicken. Conclusions: The phage vB_SalP_LDW16 is a highly lytic phage with a wide host range, which can be potentially used for preventing and treating chicken salmonellosis, as an alternative or complementary antibiotic treatment in livestock farming.