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Biocontrol of Human Pathogens in Tomato and Sprouted Seed Production using Bacterial Isolates Derived from Natural Environments


The numerous foodborne illness outbreaks linked to fresh produce continues to represent a significant food safety issue. It is well established that post-harvest washing of produce can reduce but not eliminate pathogens. Indeed, there is evidence that washing stimulates the growth of pathogens during storage due to reduction in competitive background microflora. The objective of this project is to develop a biocontrol method that can be applied pre- and post harvest to reduce or eliminate the growth of pathogens such as Salmonella on tomato fruit and sprouting seeds. The approach will be to isolate antagonistic bacteria (for example, Enterobacter and Bacillus spp) that alone or in combination can inhibit the growth of pathogens (Salmonella, E. coli O157 and Listeria monocytogenes) either through production of antimicrobial agents and/or competitive exclusion. The isolates will be recovered from various sources such as processing environments, soil and fresh produce. The synergistic activity of introducing pathogen infecting bacteriophage into the biocontrol bacterial cocktail will be assessed.
The biocontrol cocktail will be introduced onto the flowers of tomato plants along with Salmonella. The subsequent fruit will be screened for the presence of Salmonella and introduced biocontrol bacteria. In further studies tomatoes inoculated with Salmonella will be introduced into suspensions of biocontrol bacteria (mimic dunk tank water) and levels of the human pathogen monitored during post-treatment storage. Trials will be performed where seeds (mung bean, soy bean and alfalfa) inoculated with pathogens (E. coli O157:H7, L. monocytogenes and Salmonella) will be introduced into a soak solution containing biocontrol bacteria. The seeds will then be sprouted and changes in pathogen levels determined. Effects, if any, on the shelf-life of sprouts or tomatoes will be determined under post-harvest storage conditions.

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Expected Impact of Project Outcomes on Food Safety in Ontario:
Health agencies promote the increased consumption of fresh fruit and vegetables to prevent chronic diseases, in addition to improving the general health of the population. However, in recent years health advisories have been issued to warn vulnerable groups to avoid consuming certain types of produce because of food safety risks. In November 2005, one of the largest salmonellosis outbreaks within Ontario was linked to contaminated mung bean sprouts. In 2004, a further Salmonella outbreak within Ontario implicating Roma tomatoes resulted in hospitalization of 14 percent of those infected.
Preventing contamination of crops in the field is problematic due to the open nature of the field environment. Attempting to wash produce to remove field-acquired contamination is compromised by the bacteria being present within biofilms or internalized into the inner tissue. Water used to wash produce can be a significant route for disseminating pathogens. In addition, the growing organic market is reluctant to apply sanitizers due to the perceived hazards derived from chemical residues. In this regard, the proposed biocontrol method will provide an alternative approach for controlling pathogens at the primary production level and at post-harvest. To date the majority of biocontrol studies have used lactic acid bacteria or other bacterial types not specifically adapted to the plant environment. In contrast our approach will meet with more success given that the isolates will be derived from plant sources.
The current annual farm gate value of fresh produce within Ontario is valued at over $400 million. By using innovative pathogen reduction interventions, such as biocontrol based approaches, it can be envisaged that the microbiological quality of fresh produce within the province will be enhanced. This in turn will decrease the number of foodborne illness outbreaks whilst meeting consumer demand for organic produce. <P> For more information, please visit the <a href="; target="_blank">Ontario Ministry of Agriculture, Food & Rural Affairs (OMAFRA) Food Safety Research Program</a>.

Warriner, Keith
University of Guelph
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