Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/9/2012
Publication Date: 7/1/2012
Citation: Hao, L., Willis, D.K., Andrews-Polymenis, H., Mcclelland, M., Barak, J.D. 2012. Requirement of siderophore biosynthesis for plant colonization by Salmonella enterica. Applied and Environmental Microbiology. 78(13):4561-4570. Interpretive Summary: Salmonella enterica causes food contamination and subsequent human disease in a number of agronomically important crop species such as alfalfa sprouts, tomatoes and lettuce. We have discovered that the ability to utilize iron by the bacterium is important for the bacterium to colonize plants and contaminate the food supply. This discovery provides insight into potential control measures that can reduce the contamination of vegetables by Salmonella. Our finding predicts that methods to control the amount of iron available during the growth of the bacteria by interfering with the bacterial systems required for scavenging of iron could lead to significant reduction in food contamination. This research is extremely topical and is of importance to many areas of food safety including the growth and processing of vegetables.
Technical Abstract: Contaminated fresh produce has become the number one vector of non-typhoidal salmonellosis to humans. However, Salmonella enterica genes essential for the life cycle of this organism outside the mammalian host are for the most part unknown. Screening deletion mutants led to the discovery that an aroA mutant had a significant alfalfa seedling colonization defect due to a failure to replicate. AroA is part of the chorismic acid biosynthesis pathway, a central metabolic node involved in aromatic amino acid and siderophore production. Addition of tryptophan or phenylalanine to alfalfa root exudates did not restore the aroA mutant replication. However, addition of ferrous sulfate restored in planta growth of the aroA mutant, as well as alfalfa colonization. Tryptophan and phenylalanine auxotrophs had minor plant colonization defects suggesting that sub-optimal tryptophan and phenylalanine concentrations in root exudates were not major limiting factors for Salmonella replication. An entB mutant defective in siderophore biosynthesis had similar colonization and growth defects as the aroA mutant and the defective phenotype was complemented with addition of ferrous sulfate. Biosynthetic genes of each Salmonella siderophore, enterobactin and salmochelin, were up-regulated in alfalfa root exudates; yet, only enterobactin is sufficient for plant survival and persistence. Similar results in lettuce, indicate that siderophore biosynthesis is a widespread or perhaps universal plant colonization fitness factor for Salmonella, similar to other plant-associated bacteria.