Reducing tomato contamination with Salmonella through cultivar selection and maturity at harvest.

Reducing tomato contamination with Salmonella through cultivar selection and maturity at harvest.

Oct. 1, 2009 - Sep. 30, 2010

2009-79

$60,771.00

Max Teplitski, Ph.D.
University of Florida

Jason T. Noel, Ph.D.

Final Report

Recent studies demonstrate that interactions of human pathogens with crops involve specific plant and bacterial genes. Our preliminary observations that Salmonella gene expression within tomatoes depends both on the plant’s genotype and on specific tomato volatiles that differentially accumulate in fruits as they ripen. These discoveries led to a hypothesis that it should be possible to identify crop cultivars that are less susceptible to contamination with Salmonella, much as breeders select for disease-resistant crops varieties. We also expect to discover an additive effect of the genotype and maturity in affecting susceptibility of the product to contamination with Salmonella. This hypothesis will be tested with a suite of tomato-specific Salmonella gene reporters, regulation of which will be tracked in tomato fruits of different cultivars at two maturity stages. Our group was the first to develop these reporters. They provide more sensitive, reliable and quantitative information than the total Salmonella cell count. This proposed characterization of a Salmonella-“resistant” cultivar or a maturity stage will suggest a cost-effective solution for reducing the risk of proliferation of Salmonella in tomatoes. When combined with other practices (e.g. elimination of reservoirs of pathogens from processing facilities), these efforts will help ensure microbiological safety of produce. TECHNICAL ABSTRACT Contamination of vegetables most likely occurs both pre- and post-harvest (even though routes of infection and sources of pathogens in the production environment are still a matter of discussion). Can produce safety be improved without imposing further regulatory burdens or additional costs on producers? We suggest that it should be possible to identify Salmonella-“resistant” cultivars so as to minimize the contamination of produce with S. enterica, much as breeders select for disease-resistant crops varieties. Such breeding efforts require an easy screen. However, because Salmonella contaminates plants without causing visible symptoms or damage during colonization and spread, the selection of “Salmonella resistant” plant genotypes is less than straightforward. The first step to really solving this problem was to identify those bacterial genes that are crucial to the ability of Salmonella to contaminate and persist on tomatoes. With funding from Florida Tomato Committee in the past two years we characterized approximately forty Salmonella genes that are differentially regulated in tomatoes. These gene reporters can now be used for a direct and straightforward screen of the existing tomato cultivars or maturity stages that are less susceptible to contamination with Salmonella enterica. The overall objective of this proposal is to identify cultivar and fruit ripeness stage (or their combination) that may be less susceptible to contamination with Salmonella or less conducive to the growth of the pathogen. The goal of this proposal will be achieved by screening for a cultivar in which expression of the majority of Salmonella “tomato-specific” genes is strongly repressed. These discoveries will contribute to the future comprehensive sustainability and food safety program that incorporates cultivar selection and minor alteration of harvesting practices.

Technical Abstract

Contamination of vegetables most likely occurs both pre- and post-harvest (even though routes of infection and sources of pathogens in the production environment are still a matter of discussion). Can produce safety be improved without imposing further regulatory burdens or additional costs on producers? We suggest that it should be possible to identify Salmonella-“resistant” cultivars so as to minimize the contamination of produce with S. enterica, much as breeders select for disease-resistant crops varieties. Such breeding efforts require an easy screen. However, because Salmonella contaminates plants without causing visible symptoms or damage during colonization and spread, the selection of “Salmonella resistant” plant genotypes is less than straightforward. The first step to really solving this problem was to identify those bacterial genes that are crucial to the ability of Salmonella to contaminate and persist on tomatoes. With funding from Florida Tomato Committee in the past two years we characterized approximately forty Salmonella genes that are differentially regulated in tomatoes. These gene reporters can now be used for a direct and straightforward screen of the existing tomato cultivars or maturity stages that are less susceptible to contamination with Salmonella enterica. The overall objective of this proposal is to identify cultivar and fruit ripeness stage (or their combination) that may be less susceptible to contamination with Salmonella or less conducive to the growth of the pathogen. The goal of this proposal will be achieved by screening for a cultivar in which expression of the majority of Salmonella “tomato-specific” genes is strongly repressed. These discoveries will contribute to the future comprehensive sustainability and food safety program that incorporates cultivar selection and minor alteration of harvesting practices.