Specific responses of Salmonella enterica to tomato varieties and fruit ripeness identified by in vivo expression technology.

Jason T Noel,Nabil Arrach,Michael Mcclelland,Max Teplitski,Ali Alagely,Niyaz Ahmed

doi:10.1371/journal.pone.0012406

Abstract

BackgroundRecent outbreaks of vegetable-associated gastroenteritis suggest that enteric pathogens colonize, multiply and persist in plants for extended periods of time, eventually infecting people. Genetic and physiological pathways, by which enterics colonize plants, are still poorly understood.Methodology/Principal FindingsTo better understand interactions between Salmonella enterica sv. Typhimurium and tomatoes, a gfp-tagged Salmonella promoter library was screened inside red ripe fruits. Fifty-one unique constructs that were potentially differentially regulated in tomato relative to in vitro growth were identified. The expression of a subset of these promoters was tested in planta using recombinase-based in vivo expression technology (RIVET) and fitness of the corresponding mutants was tested. Gene expression in Salmonella was affected by fruit maturity and tomato cultivar. A putative fadH promoter was upregulated most strongly in immature tomatoes. Expression of the fadH construct depended on the presence of linoleic acid, which is consistent with the reduced accumulation of this compound in mature tomato fruits. The cysB construct was activated in the fruit of cv. Hawaii 7997 (resistant to a race of Ralstonia solanacearum) more strongly than in the universally susceptible tomato cv. Bonny Best. Known Salmonella motility and animal virulence genes (hilA, flhDC, fliF and those encoded on the pSLT virulence plasmid) did not contribute significantly to fitness of the bacteria inside tomatoes, even though deletions of sirA and motA modestly increased fitness of Salmonella inside tomatoes.Conclusions/SignificanceThis study reveals the genetic basis of the interactions of Salmonella with plant hosts. Salmonella relies on a distinct set of metabolic and regulatory genes, which are differentially regulated in planta in response to host genotype and fruit maturity. This enteric pathogen colonizes tissues of tomatoes differently than plant pathogens, and relies little on its animal virulence genes for persistence within the fruit.

Highlights

The increase in produce-associated gastroenteritis outbreaks indicates that non-typhoidal serovars of Salmonella enterica and enterovirulent E. coli can contaminate fruit, vegetables and sprouts [1,2,3]
Typhimurium 14028 as a model In a preliminary study, we tested whether S. enterica sv
Despite improvements in agricultural and management practices, leafy greens, tomatoes, cucurbits, peppers and nuts were among the foods linked to outbreaks of gastrointestinal illnesses caused by Escherichia coli O157:H7 and non-typhoidal serovars of Salmonella enterica, causing thousands of hospitalizations and multimillion dollar damage to the produce industry

Summary

Introduction

The increase in produce-associated gastroenteritis outbreaks indicates that non-typhoidal serovars of Salmonella enterica and enterovirulent E. coli can contaminate fruit, vegetables and sprouts [1,2,3]. Typhimurium 14028) and enterovirulent E. coli were found to colonize internal tissues of tomato, lettuce, alfalfa, cilantro, where they reach population levels as high as 105–107 cfu/g of plant tissue under field and/or laboratory conditions [4,5,6,7]. These populations are approximately two orders of magnitude lower than those reached by dedicated phytopathogens like Pectobacterium carotorovum [7]. By which enterics colonize plants, are still poorly understood

Methods

Results

Conclusion