1a. Objectives (from AD-416):
1. Concurrently evaluate the persistence of three foodborne pathogens (E. coli O157:H7, Salmonella, and Norovirus) on foliar surfaces of plants (lettuce and spinach), under the same conditions to facilitate a direct comparison of the survival characteristics of these organisms in the phyllosphere. 2. Investigate plant defense pathways during attachment and internalization of E. coli O157:H7, Salmonella and noroviruses. Determine if plants respond to these human pathogens using one or more plant-specific defense strategies. 3. Evaluate the metabolic pathways used by pathogenic bacteria to attach to plants, specifically using a novel approach to identify the major proteins of pathogenic bacteria using proteomics.
1b. Approach (from AD-416):
ARS will acquire the basic knowledge of how E. coli O157:H7, Salmonella, and norovirus survive under the exact same growing conditions, and to use culture and molecular methods to evaluate the persistence of these pathogens on foliar surfaces and plant responses to these pathogens. This information will be used by both ARS and the Cooperator to jointly develop a comprehensive understanding the interaction of bacterial and viral pathogens with leafy green crops and possibly develop antimicrobials or Good Agricultural Practices that can limit the contamination of produce. The Cooperator will specifically evaluate the infectivity of norovirus inoculated on leafy greens, and will determine methods using quantitative real time PCR (qPCR) targets to measure the expression of plant defense responses to pathogens.
3. Progress Report:
The rpoS gene in the bacterial pathogens E. coli O157:H7 and Salmonella Typhimurium may allow these organisms to survive for extended periods on leafy green commodities. Our study comparatively evaluated Salmonella and E. coli O157:H7 strains which do and do not possess the rpoS gene on spinach plants, along with the survival of murine norovirus (MNV), a surrogate for the viral pathogen norovirus. Preliminary results indicate that the rpoS gene does influence the number of cells and the duration of survival of both bacterial pathogens. The Salmonella Typhimurium strain containing the rpoS gene seems to survive for the longest duration (8 days) compared to E. coli O157:H7 (3 days). The MNV was detected for up to 8 days on the spinach plants well. All bacterial strains were inoculated into soil and recovered for up to 120 days and will be used to determine if survival and exposure of these bacterial pathogens in soil can increase their persistence on leafy green commodities.