Food Safety Research

NON-TECHNICAL SUMMARY: Life outside an animal model may be foreign to the study of human bacterial pathogens but the incidence of foodbourne illness caused by consumption of contaminated fresh produce is rising. Co-opting plants to vector itself may increase the competitiveness of Salmonella among the enteric pathogens by utilizing our industrial food chain to widely disperse itself from host to host. However, mystery surrounds this human pathogen-plant interaction in that Salmonella enterica fails to grow on plants, but salmonellosis outbreaks continue to occur from consumption of fresh produce contaminated pre-harvest. Our lab discovered a commensal relationship between the phytobacterial pathogen, Xanthomonas vesicatoria, and S. enterica in the tomato phyllosphere. When small populations of S. enterica co-colonize the tomato phyllosphere with X. vesicatoria, S. enterica can grow. This project focuses on characterizing this interaction. First, what mechanisms are employed by X. vesicatoria to survive in the phyllosphere Does the plant pathogen open stomata and thus, allow Salmonella access to a previously unavailable niche, the stomatal cavity Or does Salmonella join a X. vesicatoria biofilm Is there a relationship between the Salmonella phyllosphere and fruit populations Finally, is the interaction specific or can Salmonella grow in the tomato phyllosphere colonized by other foliar pathogens.
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APPROACH: Plant assays with plant pathogenic bacteria and S. enterica will reveal how the two bacteria interact on tomato plants, including biofilm formation and stomatal entry. Bacterial populations will be monitored and bacteria will be examined in planta by microscopy. We will expand the plant assays to include a second phytopathogen, Pseudomonas syringae pathovar tomato. Identification of these interactions and whether the symbiosis is specific to X. vesicatoria or common will highlight the significance of phytobacterial incidence on tomato as a risk factor for S. enterica contamination and the likelihood of subsequent human disease. Understanding these mechanisms will point to strategies for reduction or elimination of produce contamination and incidence of salmonellosis from produce-associated outbreaks. Finally,how S. enterica survives on plants and interacts with the plant-associated microbial community is mostly a mystery. Our approach to characterize the FUN genes is multifaceted with the goal of revealing gene function and the specific role in plant colonization. These genes were originally identified on alfalfa seedlings on which S. enterica prefers to colonize the roots. Mutants will be examined for tomato phyllosphere attachment and continued colonization capacity. Tomato plants will be dip-inoculated and S. enterica populations will be enumerated over time. We chose dip-inoculation to simulate plant contamination via sprinkler irrigation. By examination of swarming, we will identify genes important for movement on the leaf surface or from surface to interior. By testing biofilm formation, we will identify genes important for interactions with other members of the plant-associated microbiota and survival mechanisms in general on the leaf surface. Once phenotypes for the FUN gene mutants have been complemented, comparative genomics between plant and animal pathogens will be conducted by BLASTP analysis and maximum parsimony. In addition, we will construct promoter probes and examine gene expression of S. enterica cells during tomato leaf colonization via microscopy.