Human enteric pathogens in produce: un-answered ecological questions with direct implications for food safety

Salmonella enterica is a member of the plant microbiome. Growth of S. enterica in sprouting-seed exudates is rapid; however, the active metabolic networks essential in this environment are unknown. To examine the metabolic requirements of S. enterica during growth in sprouting-seed exudates, we inoculated alfalfa seeds and identified 305 S. enterica proteins extracted 24 h postinoculation from planktonic cells. Over half the proteins had known metabolic functions, and they are involved in over onequarter of the known metabolic reactions. Ion and metabolite transport accounted for the majority of detected reactions. Proteins involved in amino acid transport and metabolism were highly represented, suggesting that amino acid metabolic networks may be important for S. enterica growth in association with roots. Amino acid auxotroph growth phenotypes agreed with the proteomic data; auxotrophs in amino acid-biosynthetic pathways that were detected in our screen developed growth defects by 48 h. When the perceived sufficiency of each amino acid was expressed as a ratio of the calculated biomass requirement to the available concentration and compared to growth of each amino acid auxotroph, a correlation between nutrient availability and bacterial growth was found. Furthermore, glutamate transport acted as a fitness factor during S. enterica growth in association with roots. Collectively, these data suggest that S. enterica metabolism is robust in the germinating-alfalfa environment; that single-amino-acid metabolic pathways are important but not essential; and that targeting central metabolic networks, rather than dedicated pathways, may be necessary to achieve dramatic impacts on bacterial growth. P lants can influence both the number and species of bacteria that colonize their surfaces. The availability and types of nutrients, secretion of chemoattractants or repellents, and production of antimicrobial compounds all impact the composition of the plant-associated bacterial community. In particular, germinating seeds and roots release exudates containing complex mixtures of amino acids, sugars, organic acids, and other plant metabolites that support the growth of microbial colonists in the spermosphere and rhizosphere (1-5). Successful colonization of and persistence in these plant niches requires that bacteria acquire and metabolize these plant-derived nutrients and biosynthesize any required metabolites not sufficiently available from the plant.The compositions of seed and root exudates are dynamic and vary based on seed and plant age, soil type, temperature, nutritional status, and crop (6, 7). The identity and quantity of nutrients in plant exudates have a profound impact on bacterial growth and metabolic activity (8). Seeds that released larger quantities of carbohydrates and amino acids supported higher Enterobacter cloacae populations, rates of bacterial growth, and metabolic activity (3,4,8). Nutrient identity (e.g., alanine, arginine, and aspartate), not just type (e.g., amino acids, sugars, and lipids), influen...

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confidence: 78%

De Novo Amino Acid Biosynthesis Contributes to Salmonella enterica Growth in Alfalfa Seedling Exudates

Kwan

Pisithkul

Amador‐Noguez

et al. 2015

“…The ability of nontyphoidal strains of Salmonella enterica to colonize plants, including sprouts, alfalfa, lettuce, melons, and tomatoes, is highlighted by the rise in the number and severity of salmonellosis outbreaks linked to produce. 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).…”

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confidence: 99%

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

Moraes

Desai

Porwollik

et al. 2017

Self Cite

Human enteric pathogens, such as Salmonella spp. and verotoxigenic Escherichia coli, are increasingly recognized as causes of gastroenteritis outbreaks associated with the consumption of fruits and vegetables. Persistence in plants represents an important part of the life cycle of these pathogens. The identification of the full complement of Salmonella genes involved in the colonization of the model plant (tomato) was carried out using transposon insertion sequencing analysis. With this approach, 230,000 transposon insertions were screened in tomato pericarps to identify loci with reduction in fitness, followed by validation of the screen results using competition assays of the isogenic mutants against the wild type. A comparison with studies in animals revealed a distinct plant-associated set of genes, which only partially overlaps with the genes required to elicit disease in animals. De novo biosynthesis of amino acids was critical to persistence within tomatoes, while amino acid scavenging was prevalent in animal infections. Fitness reduction of the Salmonella amino acid synthesis mutants was generally more severe in the tomato rin mutant, which hyperaccumulates certain amino acids, suggesting that these nutrients remain unavailable to Salmonella spp. within plants. Salmonella lipopolysaccharide (LPS) was required for persistence in both animals and plants, exemplifying some shared pathogenesis-related mechanisms in animal and plant hosts. Similarly to phytopathogens, Salmonella spp. required biosynthesis of amino acids, LPS, and nucleotides to colonize tomatoes. Overall, however, it appears that while Salmonella shares some strategies with phytopathogens and taps into its animal virulence-related functions, colonization of tomatoes represents a distinct strategy, highlighting this pathogen's flexible metabolism.IMPORTANCE Outbreaks of gastroenteritis caused by human pathogens have been increasingly associated with foods of plant origin, with tomatoes being one of the common culprits. Recent studies also suggest that these human pathogens can use plants as alternate hosts as a part of their life cycle. While dual (animal/plant) lifestyles of other members of the Enterobacteriaceae family are well known, the strategies with which Salmonella colonizes plants are only partially understood. Therefore, we undertook a high-throughput characterization of the functions required for Salmonella persistence within tomatoes. The results of this study were compared with what is known about genes required for Salmonella virulence in animals and interactions of plant pathogens with their hosts to determine whether Salmonella repurposes its virulence repertoire inside plants or whether it behaves more as a phytopathogen during plant colonization. Even though Salmonella utilized some of its virulence-related genes in tomatoes, plant colonization required a distinct set of functions.KEYWORDS Salmonella, foodborne pathogens, plant-microbe interactions

“…S. enterica populations were higher on produce infected with bacterial soft rot than on produce that was mechanically damaged or affected by fungal soft rot (12). Brute force pathogens, such as those that cause soft rot, macerate tissue by overwhelming plant defenses once population levels reach a quorum (13); they can increase S. enterica survival on plants by liberating nutrients that the human pathogen cannot access on its own (4). Stealth plant pathogens (14), such as Xanthomonas spp.…”

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confidence: 99%

“…S. enterica appears to establish reservoirs on plants and to use them as vectors to animals, a preferred host (3). What factors influence S. enterica persistence in plants is not well understood (4).…”

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confidence: 99%

Xanthomonas perforans Colonization Influences Salmonella enterica in the Tomato Phyllosphere

Potnis

Soto-Arias

Cowles

et al. 2014

Salmonella enterica rarely grows on healthy, undamaged plants, but its persistence is influenced by bacterial plant pathogens. The interactions between S. enterica, Xanthomonas perforans (a tomato bacterial spot pathogen), and tomato were characterized. We observed that virulent X. perforans, which establishes disease by suppressing pathogen-associated molecular pattern (PAMP)-triggered immunity that leads to effector-triggered susceptibility, created a conducive environment for persistence of S. enterica in the tomato phyllosphere, while activation of effector-triggered immunity by avirulent X. perforans resulted in a dramatic reduction in S. enterica populations. S. enterica populations persisted at ϳ10 times higher levels in leaves coinoculated with virulent X. perforans than in those where S. enterica was applied alone. In contrast, S. enterica populations were ϳ5 times smaller in leaves coinoculated with avirulent X. perforans than in leaves inoculated with S. enterica alone. Coinoculation with virulent X. perforans increased S. enterica aggregate formation; however, S. enterica was not found in mixed aggregates with X. perforans. Increased aggregate formation by S. enterica may serve as the mechanism of persistence on leaves cocolonized by virulent X. perforans. S. enterica association with stomata was altered by X. perforans; however, it did not result in appreciable populations of S. enterica in the apoplast even in the presence of large virulent X. perforans populations. Gene-for-gene resistance against X. perforans successively restricted S. enterica populations. Given the effect of this interaction, breeding for disease-resistant cultivars may be an effective strategy to limit both plant disease and S. enterica populations and, consequently, human illness.