Purpose: The objective of this study was to investigate prophage induction as a way to control foodborne bacteria on fresh produce.
Methods: Lysogenized Escherichia coli isolates were treated with phage inducers (mitomycin C and streptonigrin, at concentrations of 0, 0.2, 0.5, 1 and 2 µg/ml), in broth culture. OD600 nm readings were taken at 0, 1.5, 3, 8 and 18 h during incubation at 37°C. PCR of prophage integrase genes was used to confirm the release of induced phages. In another experiment, the stem scar of fresh red greenhouse tomatoes was inoculated with 7.0 x 109 CFU/ml of an overnight culture of E. coli strain 185. After drying, mitomycin C (4 and 6 µg/ml) was sprayed on tomatoes, while control tomatoes were sprayed with water. Following overnight incubation, the tomatoes were immersed in PBS, the cells were collected by centrifuging the PBS, washed to remove residual inducer, and plate counts were performed.
Results: Beginning at 3 h for mitomycin C and 8 h for streptonigrin, all concentrations significantly reduced the OD of the broth cultures relative to the control in a concentration dependent manner. PCR confirmed bacterial release of prophages, and also showed differential prophage induction from the compounds. One log (4 µg/ml) and three log (6 µg/ml) reductions in E. coli cells were observed on tomatoes sprayed with mitomycin C compared to those sprayed with water.
Significance: These results demonstrate the feasibility of using prophage induction to control foodborne pathogens on fresh produce. The differential induction of phages means that multiple inducers will need to be included in an antimicrobial based on this concept.