Jan. 1, 2011 - Dec. 31, 2012Award Number
Linda J. Harris, Ph.D.
University of California, Davis
Luxin Wang, Ph.D.Resources
Salmonella spp. has been implicated in numerous outbreaks of foodborne illness tied to the consumption of fresh fruits, vegetables, and nuts, seeds and spices. Multistate outbreaks of salmonellosis due to consumption of tomatoes, mangos, melons and raw almonds have highlighted the ability of Salmonella to persist in a wide range of pre- and postharvest environments. Exposure to large swings in moisture, temperature, and nutrient levels are
expected in these environments. The relative tolerance to these conditions is known differ among strains of Salmonella. In addition, some of the environmental stressors may trigger a variety of survival response mechanisms in some strains providing further competitive advantage. While strain dependent survival phenomena have been documented, the mechanism of these differences is not clear. The proposed research seeks to increase our understanding of the environmental factors that trigger survival mechanisms in outbreak-related strains of Salmonella and to better elucidate those mechanisms related to desiccation tolerance and environmental persistence. The results will help the produce industry to better interpret Salmonella-positive test results and should assist in making informed decisions related to pre and postharvest risks of contamination.
Salmonella spp. has been implicated in numerous outbreaks of foodborne illness tied to the consumption of fresh produce. Multistate outbreaks of salmonellosis due to consumption of tomatoes, mangos, melons and raw almonds have highlighted the ability of Salmonella to persist in a wide range of pre- and postharvest environments. Exposure to large swings in moisture, temperature, and nutrient levels are expected in these environments. Introduction of Salmonella may occur at any point in the farm-to-fork continuum and the contamination matrix may be in one of many forms: dry (e.g., dust), wet (e.g., decaying material), solid (e.g., food-contact surface), liquid (e.g., water). Our preliminary data has demonstrated that Salmonella generally survive better during desiccation when cultured on solid agar surfaces than when cultured in liquid medium; desiccation tolerance differs among strains of Salmonella; desiccation-tolerant strains grown on agar surfaces are morphological different from cells cultured in liquid and this morphological difference (formation of multicellular structures) is related to differences in production of cellulose and fimbriae (rdar morphology); outbreak-associated strains are consistently capable of expressing the rdar phenotype. Using almonds and to a lesser extent tomatoes as model commodities, the objectives of this study are: to evaluate the impact of pre and post-harvest environmental factors on the formation of aggregative fimbriae and cellulose; and to characterize the role of aggregative fimbriae and cellulose in the desiccation tolerance and
long-term survival of Salmonella and in acid tolerance and resistance to chlorine. We will take a multipronged approach to achieve these objectives including observation of multicellular formation and direct measurement of cellulose, fimbriae, and corresponding RNA transcripts. Rdar- constructs of Salmonella Enteritidis PT30 will be made and physiological state and stress response differences among the knockout mutants and the wild type will be determined. The roles of cellulose and fimbriae on survival of Salmonella during desiccation, long-term environmental persistence and acid tolerance will be evaluated. Understanding the phenotypic and genotypic traits that contribute to desiccation tolerance will help the produce industry to evaluate the significance of Salmonella-positive test results. Identification of those environmental factors that trigger production of protective compounds could assist in making informed decisions related to pre- and postharvest contamination risks. In addition, these data will be important for development (strain selection, culture conditions) and interpretation of scientific data related to produce and nuts (e.g., challenge studies or validation protocols).