| Contaminated fresh produce has become the number one vector of Salmonella enterica to humans. However, genes and metabolic pathways that are essential for the life cycle of the pathogen in the plant environments are poorly characterized. To identify what genes contribute to S. enterica plant colonization, I screened a library of deletion mutants and discovered two genes that play important roles during S. enterica interaction with plants.;An aroA mutant was found to have significant root colonization defects, due to a failure to replicate in the root exudates. aroA is part of the chorismic acid biosynthesis pathway, a central metabolic node involved in aromatic amino acid and siderophore production. Examination of each of the downstream pathways of chorismic acid biosynthesis led to the discovery that iron is a major limiting factor and S. enterica requires the production of siderophore to grow in the root exudates, while the availability of aromatic amino acids plays a minor role. Biosynthetic genes of each Salmonella siderophore, enterobactin and salmochelin, were upregulated in alfalfa root exudates, yet only enterobactin was required for S. enterica to survive and persist in planta. A similar result in lettuce leaves indicates that siderophore biosynthesis is a widespread or perhaps universal plant colonization fitness factor for Salmonella.;In a separate project, I characterized the role of a yobG in S. enterica plant colonization. Deletion of yobG led to significant root colonization defects. Transcriptional reporter assays revealed that yobG negatively regulates the transcription of the PmrA/B two- component system (TCS) through the PhoP/Q system. In addition, yobG negatively regulates the transcription of the KdpD/E TCS, independent of the PhoP/Q system. The PmrA/B system has a major role in polymyxin resistance and deletion of yobG increased bacterial resistance to polymyxin. A deletion of pmrF, which is in an operon directly regulated by the PmrA/B independent of the PhoP/Q system, had alfalfa root colonization defect. However, deletion of either phoPQ, or pmrD , which is only regulated by the PhoP/Q system, had no effect. This suggests that the PmrA/B, not the PhoP/Q system plays an important role in S. enterica plant colonization. |
