Investigating behaviors of zoonotic and human pathogens on plants

Markland, Sarah Megan
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University of Delaware
The Centers for Disease Control and Prevention (CDC) estimates the incidence of foodborne illness attributed to fruit and vegetable consumption at three million cases in the U.S. annually (Painter et al. , 2013). The means by which food crops become contaminated with foodborne pathogens as well as how these organisms persist within the phyllosphere and rhizosphere of these plants is an extremely complex issue. In addition, the CDC estimates that unspecified agents are responsible for 38.4 million out of the 48 million (80 %) cases of foodborne illnesses. It is hypothesized that environmental E. coli not typically associated with the ability to cause disease in humans could potentially be responsible for some of these cases. The role of avian pathogenic E. coli (APEC) in human disease remains questionable, although genomic evaluations of these isolates suggest the ability of these extraintestinal pathogenic E. coli (EXPEC) to cause intestinal illnesses as well as foodborne urinary tract infections (FUTIs) in humans. In these studies we demonstrate the enhanced ability of avian pathogenic E. coli (APEC) to persist on leafy greens indicating enhanced environmental fitness in these isolates and potential risks to human health. Outbreaks associated with food crops continue to occur and it is clear that different approaches are critical to enhance the safety of these foods. Within the last few years the fields of food safety and plant science have begun to merge to more efficiently address some of the knowledge gaps involving the mechanisms by which human pathogens contaminate plants. We currently know a great deal about the mechanisms by which plant pathogens are able to cause disease in plant crops as well as the immune response of these plants to these pathogens. By studying the relationships between plant pathogens and the plant immune response, scientists have been able to successfully develop biocontrol strategies to reduce crop damage attributed to plant disease. In these studies we also demonstrate the benefits of a plant growth promoting rhizobacteria (PGPR), Bacillus subtilis UD1022, and its ability to increase the plant immune response to reduce the persistence of Listeria on lettuce plants as well as its inhibitory properties towards L. monocytogenes on cantaloupe rind. In addition to studying plant-microbe interactions between plants and human bacterial pathogens, it is also important to address plant associations with human viruses. Human norovirus is the leading cause of foodborne illness worldwide with the majority of outbreaks linked to fresh produce and leafy greens. It is essential that we also thoroughly understand the type of relationship and interactions that take place between plants and human norovirus in order to better utilize control strategies to reduce transmission of norovirus in the field onto plants harvested for human consumption. In these studies the expression of gene markers for the salicylic acid and jasmonic acid plant defense pathways were measured and compared in romaine lettuce ( Lactuca sativa ) and Arabidopsis thaliana Col-0 plants were inoculated with either MNV, TV, human norovirus GII.4 or HBSS (control). The results of these studies suggest that the jasmonic acid pathway of plant defense is likely involved in the plant immune response to human norovirus. This research provides the first pieces of information regarding how foodborne viruses interact with plants in the pre-harvest environment.