Control of Bacterial Foodborne Pathogens on Fresh Produce: A Trojan Horse Tale

Introduction: Fresh produce is increasingly implicated in foodborne outbreaks.  Most fresh produce are consumed raw emphasizing the need to develop non-thermal methods to control foodborne pathogens.  Temperate bacteriophages are found within genomes of various bacterial foodborne pathogens.  These prophages can be induced, resulting in lysis of their host.

Purpose: The goal of this study was to investigate induction of prophages as a novel strategy to control bacterial pathogens on fresh produce.

Methods: Growth of various lysogenized foodborne pathogens exposed to a phage inducer (0 or 2 µg/ml mitomycin C) was monitored by OD₆₀₀.  Release of induced prophages from their host was confirmed by PCR to detect phage-specific integrases.  The ability of mitomycin C to induce prophages in Escherichia coli O157:H7 and Salmonella Duesseldorf on fresh produce was evaluated by inoculating the stem scar of red greenhouse tomatoes or spinach leaves with 5×10⁷ and 5×10⁸ cells, respectively.  After drying, mitomycin C (6µ/ml) was sprayed onto each sample, while control samples were sprayed with water.  Following overnight incubation, the bacterial cells were recovered and plate counts were performed.

Results: Beginning at 3 hours after addition of mitomycin C, growth of E. coli O157:H7, Salmonella spp. and Listeria monocytogenes strains resulted in a marked decrease in OD₆₀₀.  PCR confirmed release of prophages in these lysates.  Growth of certain strains, including Salmonella Tyresoe shown to contain no phages, resulted in only a minor decrease in OD₆₀₀ after addition of mitomycin C, suggesting lack of induction.  A three-log reduction in Salmonella Duesseldorf and E. coli O157:H7 was observed on tomatoes sprayed with mitomycin C compared to those sprayed with water, while a one-log reduction in E. coli O157:H7 was obtained on spinach.

Significance: These findings serve as a proof of concept demonstrating that prophage induction can efficiently control bacterial foodborne pathogens on fresh produce.