Defining The Growth and Survival Mechanisms of The Human Pathogen Salmonella Typhi in Plants

Most of the other 2,600 nontyphoidal Salmonella serovars infect a broad range of mammalian hosts including both humans and animals, and some infect even plants. However, S. Typhi - the cause of human typhoid fever - is known to be constrained solely to humans. Surprisingly however, we found that S. Typhi is able to grow more efficiently in plants than in humans, and despite the massive S. Typhi burden, the infected plants do not show discernible phenotypic change, implying that S. Typhi contaminated produce is difficult to detect, and can be one of the major sources of typhoid fever transmission and outbreak. Our central hypothesis is that crops are asymptomatic permissive hosts for S. Typhi replication. To test the hypothesis, we have adopted a well-established plant model system - tomato - for studying the molecular and cellular mechanism(s) of plant-pathogen interactions. Following two objectives are proposed.Objective 1. Investigate whether S. Typhi's known virulence genes
essential during the human infectious cycle are also important during the plant infectious cycle. In particular we will examine whether PhoPQ (a global regulator for virulence gene expression), Vi polysaccharide capsule (for survival by avoiding pathogen associated molecular patterns PAMPs-associated immunity), Type 3 secretion system SPI1 (for infection) and SPI2 (for replication and survival), and typhoid toxin (associated with disease syndromes and persistent infection) also play an important role during its tomato infectious cycle.Objective 2. Identify and characterize S. Typhi's virulence genes contributing to the massive replication and survival of S. Typhi in plants. Using the comprehensive S. Typhi mutant library that is in place, we will conduct high throughput screening analysis via barcoding of the S. Typhi genomes recovered at various time points important for studying genes for replication and/or survival, which will be followed by next-generation sequencing and data
analysis. Promising bacterial genes will be followed up by mutational analysis, and characterized in the context of the tomato infection. The role(s) of the validated bacterial genes will be determined by identifying their host target in plants.Overall, we seek to understand how similarly as well as differently S. Typhi responds within the two permissive hosts, humans and plants, by comparing bacterial tactics to replicate and survive in plants to those in humans, as S. Typhi infection in human cells are well understood. The proposed investigations will provide insights into better controlling Salmonella-contaminated produce, but also developing a pathogen resistant variety of plants.