Isolation and Characterization of a Virulent Bacteriophage for Controlling Salmonella Enteritidis Growth in Ready-to-Eat Mixed-Ingredient Salads

Recently, ready-to-eat vegetable salads have gained popularity worldwide. However, the microbial safety of ready-to-eat salads is a health concern, primarily due to Salmonella enteritidis contamination during the growing, harvesting, processing, and handling of produce. This study aimed to develop a bacteriophage-based strategy to control S. enteritidis growth in mixed-ingredient salads. A lytic Salmonella-specific phage SapYZU01 was isolated from a soil sample from a suburban vegetable field in Yangzhou, China. SapYZU01 exhibited characteristics such as a short latent period, a large burst size, and a lytic effect against 13 S. enteritidis strains isolated from various sources (human, pork, deli, chicken, and chicken meat). The SapYZU01 genome did not contain virulence or antibiotic resistance genes. SapYZU01 significantly decreased the viability of S. enteritidis cells in iceberg lettuce, chicken meat, and mixed-ingredient (lettuce+chicken) salads at 37 and 25 °C. Furthermore, bacterial counts in the salad decreased significantly (by 4.0 log colony-forming units (CFU)/g) at 25 °C upon treatment of contaminated lettuce with SapYZU01 at an MOI of 100 prior to salad preparation. Bacterial counts were decreased by 3.8 log CFU/g at 25 °C in (lettuce+chicken) salads treated with SapYZU01 at an MOI of 100 after the salad preparation. In contrast, treating cooked chicken meat with SapYZU01 at an MOI of 100 before mixing it with contaminated lettuce decreased the bacterial count of the salad by 1.2 log CFU/g at 25 °C. These findings indicate the potential application of SapYZU01 as a natural biocontrol agent against S. enteritidis in mixed-ingredient salads. Furthermore, the application of lytic bacteriophage SapYZU01 in mix-ingredient salads should considered the bacteriophage treatment method in addition to the bacteriophage concentration.