Specific Responses of Salmonella enterica to Tomato Varieties and Fruit Ripeness Identified by In Vivo Expression Technology

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.

Effects of nitrogen and potassium fertilization on the susceptibility of tomatoes to post-harvest proliferation of Salmonella enterica

“…The main effects of the tomato cultivar (F ¼ 0.62, P ¼ 0.432) and Salmonella strain (F ¼ 0.60, P ¼ 0.4386) were not significant, while the main effects of the harvest time (F ¼ 351.80, P < 0.0001) and tomato maturity (F ¼ 96.13, P < 0.0001) were highly significant. The effects of tomato maturity on Salmonella growth inside tomatoes are consistent with the reports that red tomatoes were significantly more conducive to proliferation of Salmonella than green tomatoes (Shi et al, 2007; and that the gene expression in Salmonella changes in response to specific metabolites present within immature tomato fruit Noel et al, 2010). Statistically significant two-way and three-way interactions were as follows: potassium fertilization levels  harvest time (F ¼ 2.38, P ¼ 0.027), Salmonella strain  harvest time (F ¼ 9.03, P < 0.0001), tomato maturity  harvest time (F ¼ 10.41, P < 0.0001), nitrogen fertilization levels  potassium fertilization levels  harvest time (F ¼ 1.84, P ¼ 0.037), tomato cultivar  nitrogen fertilization level  harvest time (F ¼ 2.50, P ¼ 0.021), tomato maturity  nitrogen fertilization levels  harvest time (F ¼ 2.10, P ¼ 0.014), and tomato maturity  tomato cultivar  harvest time (F ¼ 4.33, P ¼ 0.0002), respectively (for the results of all F-tests, see Table 1).…”

Salmonella Persistence in Tomatoes Requires a Distinct Set of Metabolic Functions Identified by Transposon Insertion Sequencing

“…The capacity of S. enterica (and other enteric pathogens) to readily multiply within plant tissues led to the hypothesis that persistence on plants is a part of the Salmonella life cycle, serving as reservoirs prior to reinfection of the preferred animal hosts (2)(3)(4)(5). Several studies have dissected the molecular basis of plant-Salmonella interactions (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16). However, the entire complement of genetic functions required for plant colonization by Salmonella is not yet known.…”

“…It involves major changes in the bacterial transcriptome and requires genes involved in amino acid biosynthesis and transport, cellulose production, fimbriae, regulators, and surface structures (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17). Some of the same mechanisms were involved in the colonization of both vegetative and reproductive tissues on different plant species.…”

“…Some of the same mechanisms were involved in the colonization of both vegetative and reproductive tissues on different plant species. Colonization of the tomato pericarp involves differential regulation of at least 50 genes (9), some of which were also required for proliferation, including the gene cysB, encoding a cysteine metabolism regulator (9). Surface structures, like the O-antigen capsule and curli fimbriae, were also required for the colonization of tomatoes, as indicated by reduced fitness of the corresponding Salmonella mutants (11,17).…”

Robust cropping systems to tackle pests under climate change. A review

“…On tomato, S. enterica has been reported to move systemically reaching fruits and seeds from leaves (Gu et al 2011). Once within the fruits, the pathogen can multiply to high densities (Gu et al 2011;Noel et al 2010). A recent study demonstrated that Escherichia coli can internally colonize both leaves and roots of lettuce and spinach (Wright et al 2013).…”