Ethylene signalling affects susceptibility of tomatoes to Salmonella

The Tomato Variety Affects the Survival of Shigella flexneri 2a in Fruit Pericarp

The presence of enteric pathogens in produce can serve as a significant means of transmitting infections to consumers. Notably, tomatoes, as a type of produce, have been implicated in outbreaks caused by various human pathogens, such as Salmonella enterica and pathogenic Escherichia coli. However, the survival characteristics of Shigella spp. in tomatoes have not been thoroughly investigated. In this study, we assess the survival of S. flexneri 2a in two distinct varieties of post-harvested tomatoes. S. flexneri 2a was used to inoculate both regular-sized Vine tomatoes and cherry-type Mini Plum tomatoes. Our findings reveal no significant difference in Shigella survival in the pericarp of both varieties on day 2 post-inoculation. However, a significant disparity emerges on day 6, where all recovered Shigella colonies exclusively belong to the Mini Plum variety, with none associated with the Vine type. When Shigella was inoculated into the locular cavity (deep inoculation), no significant difference between varieties was observed. Additionally, we investigate the potential role of the SRL pathogenicity island (SRL PAI) in the survival and fitness of S. flexneri 2a in post-harvested tomatoes. Our results indicate that while the SRL PAI is not linked to the survival of the strains in tomato, it does impact their fitness. These findings underscore the variability in Shigella strains' survival capabilities depending on the tomato variety, highlighting the importance of understanding Shigella ecology beyond the human host and identifying molecular determinants influencing bacterial survival to mitigate the risk of future outbreaks. The significance of this data on Shigella persistence in fresh vegetables should not be underestimated, as even a small number of Shigella cells can pose a threat to the health of individuals.

Pre-heated blades for harvesting baby-leaves reduce the risk of Escherichia coli internalization in leaves

BACKGROUND

Pathogenic enterobacteria can travel through the plant vascular bundles by penetrating from cuts and persisting into ready-to-eat leafy greens. Because the cutting site is the main point of entrance and uptake, we tested how different cutting strategies can reduce bacterial internalization in leaves. Horizontal cuts at the base of the leaves were performed with two different types of tools: the first with a scalpel (by pulling the blade) and the second with a scissor-action that has blades that cuts by gliding against a thicker blade. Scissor-action generally makes closer border cuts. Blades of both types of tools have worked at 25 °C and 200 °C. The present study aimed to determine how these different types of cuts and temperatures affected bacterial uptake in leaves. Experiments were repeated on different plant genotypes and at different wilting stages.

RESULTS

Our findings showed that cutting baby-leaves with a scissor action at 200 °C significantly reduced the bacterial uptake compared to the not heated (which simulates a mechanized lettuce harvester). The most effective cutting treatments for reducing bacterial uptake were in the order: scissor 200 °C > scissor 25 °C > scalpel 200 °C > scalpel 25 °C. The scissor heated at 200 °C also prevented bacterial uptake on wilted baby-leaves.

CONCLUSION

The findings of the present study could provide a further contribution in terms of safety during harvest and suggest that a pre-heated blade supports safety during harvest of leafy greens. © 2022 Society of Chemical Industry.

Adaptation of Commensal Escherichia coli in Tomato Fruits: Motility, Stress, Virulence

Food contamination can be a serious concern for public health because it can be related to the severe spreading of pathogens. This is a main issue, especially in the case of fresh fruits and vegetables; indeed, they have often been associated with gastrointestinal outbreak events, due to contamination with pathogenic bacteria. However, little is known about the physiological adaptation and bacterial response to stresses encountered in the host plant. Thus, this work aimed to investigate the adaptation of a commensal E. coli strain while growing in tomato pericarp. Pre-adapted and non-adapted cells were compared and used to contaminate tomatoes, demonstrating that pre-adaptation boosted cell proliferation. DNA extracted from pre-adapted and non-adapted cells was sequenced, and their methylation profiles were compared. Hence, genes involved in cell adhesion and resistance against toxic compounds were identified as genes involved in adaptation, and their expression was compared in these two experimental conditions. Finally, pre-adapted and non-adapted E. coli were tested for their ability to resist the presence of toxic compounds, demonstrating that adaptation exerted a protective effect. In conclusion, this work provides new information about the physiological adaptation of bacteria colonizing the tomato fruit pericarp.

Microbiological food safety considerations in shelf-life extension of fresh fruits and vegetables

There are a number of opportunities for reducing loss and waste, and extending shelf life of fresh produce that go beyond cold chain optimization. For example, plant genotype (including ripening-related genes), presence of phytopathogens, maturity at harvest, and environmental conditions close to the harvest time, storage conditions, and postharvest treatments (washing, cutting, and waxing) all impact both shelf life of produce and food safety outcomes. Therefore, loss can be reduced and shelf life of fresh produce can be extended with plant breeding to manipulate ripening-related traits, or with pre- and postharvest treatments delaying senescence and decay. Food safety considerations of these applications are discussed.

Role of Stress-Induced Proteins RpoS and YicC in the Persistence of Salmonella enterica subsp. enterica Serotype Typhimurium in Tomato Plants

Understanding the functional role of bacterial genes in the persistence of Salmonella in plant organs can facilitate the development of agricultural practices to mitigate food safety risks associated with the consumption of fresh produce contaminated with Salmonella spp. Our study showed that Salmonella enterica subsp. enterica serotype Typhimurium (strain MDD14) persisted less in inoculated tomato plants than other Salmonella Typhimurium strains tested (JSG210, JSG626, JSG634, JSG637, JSG3444, and EV030415; P < 0.01). In-vitro assays performed in limited-nutrient conditions (growth rate, biofilm production, and motility) were inconclusive in explaining the in-planta phenotype observed with MDD14. Whole-genome sequencing combined with non-synonymous single nucleotide variations analysis was performed to identify genomic differences between MDD14 and the other Salmonella Typhimurium strains. The genome of MDD14 contained a truncated version (123 bp N-terminal) of yicC and a mutated version of rpoS (two non-synonymous substitutions, i.e., G66E and R82C), which are two stress-induced proteins involved in iron acquisition, environmental sensing, and cell envelope integrity. The rpoS and yicC genes were deleted in Salmonella Typhimurium JSG210 with the Lambda Red recombining system. Both mutants had limited persistence in tomato plant organs, similar to that of MDD14. In conclusion, we demonstrated that YicC and RpoS are involved in the persistence of Salmonella in tomato plants in greenhouse conditions and, thus, could represent potential targets to mitigate persistence of Salmonella spp. in planta. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Foliar Roughness and Water Content Impact on Escherichia coli Attachment in Baby Leafy Greens

Understanding the relation between the susceptibility of different leafy greens to human pathogen contamination and leaf traits can contribute to increase the food safety of the fresh vegetable industry. The aim of this research was to evaluate the susceptibility to E. coli ATCC 35218 attachment in 30 accessions of baby leaves, and to identify leaf traits potentially involved in the contamination. The accessions were surface inoculated with a bacterial suspension containing 1 × 10⁷ cells/mL and the attachment was measured 1.5 h after inoculation. Significant differences in attachment were detected between the accessions for p ≤ 0.05. The three most and the three least susceptible accessions were selected and characterized for leaf micro-morphological traits (stomata density and size, surface roughness) and water content. Scanning electron microscopy was used to analyse the stomatal parameters. Roughness was measured by an innovative portable 3D digital microscope. No significant correlation between the attachment of E. coli ATCC 35218 and stomatal parameters was detected, while the attachment was positively correlated with roughness and water content. The E. coli ATCC 35218 population in surface-inoculated leaves was also measured after a UV treatment, which was found to be less effective in reducing bacterial contamination in the rougher leaves. This result suggested that roughness offers UV protection, further highlighting its impact on the microbiological safety of baby leafy greens.

Food Safety Risks of Harvesting Dropped and Drooping Produce: A Review

ABSTRACT

The Produce Safety Rule of the Food Safety Modernization Act (FSMA) sets forth minimum standards for fruit and vegetable production in the United States. One provision states that growers must not harvest dropped produce because damage or ground contact may contaminate produce. In an unpublished survey of 2020 food safety inspections conducted by the Northeast Center to Advance Food Safety, handling of dropped produce covered by the FSMA was a common misunderstood and noncompliance issue among growers in the Northeast. In consideration of this provision's on-farm practicality, this review was conducted to evaluate the risks associated with dropped and drooping produce, to guide growers in making informed risk management decisions, and to answer the following questions: (i) what are the risk factors that influence transferability of pathogens from touching the ground to produce and (ii) what are the risks associated with harvesting dropped or drooping produce covered under the Produce Safety Rule? A search of online databases revealed 12 relevant publications, which highlighted moisture, contact time, and crop features as affecting contamination rates from a ground surface to a crop surface. Soil and mulch posed a differential risk, with bare soil generally presenting a lower risk than plastic mulch. The effects of other mulch types are unclear. Mulches may promote pathogen persistence in soil, although they may also protect produce from contaminated soils. These studies were limited in their scope and applicability and most did not directly address dropped produce. Research is needed to clarify the various effects of dropped and drooping produce, the impact of ground surface type on pathogen survivability and transfer, soil and crop features that facilitate contamination, and postharvest risks of harvesting dropped or drooping produce. A comprehensive understanding of these issues will guide growers in implementing preventive measures and better managing risk in a way practicable to each farm's unique conditions.

HIGHLIGHTS

Plant Bioactive Compounds as an Intrinsic and Sustainable Tool to Enhance the Microbial Safety of Crops

Outbreaks of produce-associated foodborne illness continue to pose a threat to human health worldwide. New approaches are necessary to improve produce safety. Plant innate immunity has potential as a host-based strategy for the deactivation of enteric pathogens. In response to various biotic and abiotic threats, plants mount defense responses that are governed by signaling pathways. Once activated, these result in the release of reactive oxygen and nitrogen species in addition to secondary metabolites that aim at tempering microbial infection and pest attack. These phytochemicals have been investigated as alternatives to chemical sanitization, as many are effective antimicrobial compounds in vitro. Their antagonistic activity toward enteric pathogens may also provide an intrinsic hurdle to their viability and multiplication in planta. Plants can detect and mount basal defenses against enteric pathogens. Evidence supports the role of plant bioactive compounds in the physiology of Salmonella enterica, Escherichia coli, and Listeria monocytogenes as well as their fitness on plants. Here, we review the current state of knowledge of the effect of phytochemicals on enteric pathogens and their colonization of plants. Further understanding of the interplay between foodborne pathogens and the chemical environment on/in host plants may have lasting impacts on crop management for enhanced microbial safety through translational applications in plant breeding, editing technologies, and defense priming.

Impact of Plant Pathogen Infection on Salmonella enterica subsp. enterica Serotype Typhimurium Persistence in Tomato Plants

ABSTRACT

We investigated whether the co-occurrence of phytopathogens (Clavibacter michiganensis subsp. michiganensis [Cmm] and Xanthomonas gardneri [Xg]) frequently encountered in tomato production and Salmonella enterica subsp. enterica serotype Typhimurium (strain JSG626) affects the persistence of these pathogens in tomato plant tissues during the early stages of plant growth. Cmm increased the recovery of Salmonella Typhimurium (up to 1.8 log CFU per plant at 21 days postinoculation [DPI]) from coinoculated tomato plants compared with plants inoculated with Salmonella Typhimurium alone (P < 0.05). Xg had no effect on Salmonella Typhimurium persistence in the plants. Increased persistence of Salmonella Typhimurium was also observed when it was inoculated 7 days after Cmm inoculation of the same plant (P < 0.05). In contrast, Salmonella Typhimurium reduced the population of both Cmm and Xg (up to 1.5 log CFU per plant at 21 DPI; P < 0.05) in coinoculated plants compared with plants inoculated with Cmm or Xg alone. The Xg population increased (1.16 log CFU per plant at 21 DPI; P < 0.05) when Salmonella Typhimurium was inoculated 7 days after Xg inoculation compared with plants inoculated with Xg alone. Our findings indicate that the type of phytopathogen present in the phyllosphere and inoculation time influence the persistence of Salmonella Typhimurium JSG626 and its interactions with phytopathogens cocolonized in tomato plants. Salmonella reduced the phytopathogen load in plant tissues, and Cmm enhanced the recovery of Salmonella from the coinoculated plant tissues. However, further investigations are needed to understand the mechanisms behind these interactions.

HIGHLIGHTS

Frontiers in Plant Breeding: Perspectives for the Selection of Vegetables Less Susceptible to Enteric Pathogens

Fresh vegetables including baby greens, microgreens, and sprouts can host human pathogens without exhibiting any visible signs of spoilage. It is clear that the vast majority of foodborne disease outbreaks associated with vegetable produce are not simply a result of an oversight by a producer, as it was shown that zoonotic pathogens from Enterobacteriaceae can contaminate produce through various routes throughout the entire production cycle. In this context, phenotypic and genotypic signatures have been used since early ages in agriculture to obtain better produce, and can be used today as a strategy to reduce the risk of outbreaks through plant breeding. In this mini-review, we provide an updated view and perspectives on to what extent the selection of biological markers can be used to select safer cultivars of vegetable crops such as tomato (the most studied), leafy greens and cabbage. Once this knowledge will be better consolidated, these approaches should be integrated into the development of comprehensive farm-to-fork produce safety programs.

Human Pathogen Colonization of Lettuce Dependent Upon Plant Genotype and Defense Response Activation

Fresh produce contaminated with human pathogens may result in foodborne disease outbreaks that cause a significant number of illnesses, hospitalizations, and death episodes affecting both public health and the agribusiness every year. The ability of these pathogens to survive throughout the food production chain is remarkable. Using a genetic approach, we observed that leaf colonization by Salmonella enterica serovar Typhimurium 14028s (S. Typhimurium 14028s) and Escherichia coli O157:H7 was significantly affected by genetic diversity of lettuce (Lactuca sativa L. and L. serriola L.). In particular, there was a significant variation among 11 lettuce genotypes in bacterial attachment, internalization, and apoplastic persistence after surface- and syringe-inoculation methods. We observed a significant correlation of the bacterial leaf internalization rate with stomatal pore traits (width and area). Moreover, bacterial apoplastic populations significantly decreased in 9 out of 11 lettuce genotypes after 10 days of surface inoculation. However, after syringe infiltration, populations of E. coli O157:H7 and S. Typhimurium 14028s showed positive, neutral, or negative net growth in a 10-day experimental period among seedlings of different lettuce types. The relative ability of the bacteria to persist in the apoplast of lettuce genotypes after syringe inoculation was minimally altered when assessed during a longer period (20 days) using 3.5- to 4-week-old plants. Interestingly, contrasting bacterial persistence in the lettuce genotypes Red Tide and Lollo Rossa was positively correlated with significant differences in the level of reactive oxygen species burst and callose deposition against S. Typhimurium 14028s and E. coli O157:H7 which are related to plant defense responses. Overall, we characterized the genetic diversity in the interaction between lettuce genotypes and enterobacteria S. Typhimurium 14028s and E. coli O157:H7 and discovered that this genetic diversity is linked to variations in plant immune responses towards these bacteria. These results provide opportunities to capitalize on plant genetics to reduce pathogen contamination of leaves.

Breeding Crops for Enhanced Food Safety

An increasing global population demands a continuous supply of nutritious and safe food. Edible products can be contaminated with biological (e.g., bacteria, virus, protozoa), chemical (e.g., heavy metals, mycotoxins), and physical hazards during production, storage, transport, processing, and/or meal preparation. The substantial impact of foodborne disease outbreaks on public health and the economy has led to multidisciplinary research aimed to understand the biology underlying the different contamination processes and how to mitigate food hazards. Here we review the knowledge, opportunities, and challenges of plant breeding as a tool to enhance the food safety of plant-based food products. First, we discuss the significant effect of plant genotypic and phenotypic variation in the contamination of plants by heavy metals, mycotoxin-producing fungi, and human pathogenic bacteria. In addition, we discuss the various factors (i.e., temperature, relative humidity, soil, microbiota, cultural practices, and plant developmental stage) that can influence the interaction between plant genetic diversity and contaminant. This exposes the necessity of a multidisciplinary approach to understand plant genotype × environment × microbe × management interactions. Moreover, we show that the numerous possibilities of crop/hazard combinations make the definition and identification of high-risk pairs, such as Salmonella-tomato and Escherichia coli-lettuce, imperative for breeding programs geared toward improving microbial safety of produce. Finally, we discuss research on developing effective assays and approaches for selecting desirable breeding germplasm. Overall, it is recognized that although breeding programs for some human pathogen/toxin systems are ongoing (e.g., Fusarium in wheat), it would be premature to start breeding when targets and testing systems are not well defined. Nevertheless, current research is paving the way toward this goal and this review highlights advances in the field and critical points for the success of this initiative that were discussed during the Breeding Crops for Enhanced Food Safety workshop held 5-6 June 2019 at University of California, Davis.

Salmonella adapts to plants and their environment during colonization of tomatoes

Humans and animals are considered typical hosts for Salmonella, however, also plants can be colonized. Tomatoes were linked to salmonellosis outbreaks already on several occasions. The aim of this study was, therefore, to establish a comprehensive view on the interaction between Salmonella enterica and tomatoes, and to test the hypothesis that colonization of plants is an interactive process. We assessed the persistence of Salmonella in agricultural soil, the colonization pattern in and on tomatoes, as well as the reciprocal responses of tomatoes to different Salmonella strains and Salmonella to root exudates and tomato-related media. This study revealed that Salmonella can persist in the soil and inside the tomato plant. Additionally, we show that Salmonella strains have particular colonization pattern, although the persistence inside the plant differs between the tested strains. Furthermore, the transcriptome response of tomato showed an up-regulation of several defense-related genes. Salmonella transcriptome analysis in response to the plant-based media showed differentially regulated genes related to amino acid and fatty acid synthesis and stress response, while the response to root exudates revealed regulation of the glyoxylate cycle. Our results indicate that both organisms actively engage in the interaction and that Salmonella adapts to the plant environment.

Genome-Wide Comparative Functional Analyses Reveal Adaptations of Salmonella sv. Newport to a Plant Colonization Lifestyle

Outbreaks of salmonellosis linked to the consumption of vegetables have been disproportionately associated with strains of serovar Newport. We tested the hypothesis that strains of sv. Newport have evolved unique adaptations to persistence in plants that are not shared by strains of other Salmonella serovars. We used a genome-wide mutant screen to compare growth in tomato fruit of a sv. Newport strain from an outbreak traced to tomatoes, and a sv. Typhimurium strain from animals. Most genes in the sv. Newport strain that were selected during persistence in tomatoes were shared with, and similarly selected in, the sv. Typhimurium strain. Many of their functions are linked to central metabolism, including amino acid biosynthetic pathways, iron acquisition, and maintenance of cell structure. One exception was a greater need for the core genes involved in purine metabolism in sv. Typhimurium than in sv. Newport. We discovered a gene, papA, that was unique to sv. Newport and contributed to the strain’s fitness in tomatoes. The papA gene was present in about 25% of sv. Newport Group III genomes and generally absent from other Salmonella genomes. Homologs of papA were detected in the genomes of Pantoea, Dickeya, and Pectobacterium, members of the Enterobacteriacea family that can colonize both plants and animals.

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

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.

Laboratory Activity to Teach about the Proliferation of Salmonella in Vegetables

We designed a three-week laboratory experience that can complement any microbiology teaching laboratory to expand students' knowledge of the ecology of human enteric pathogens outside of their animal hosts. Through their participation in this laboratory activity, students learned that vegetative and reproductive plant parts could be a natural habitat for enteric bacteria such as non-typhoidal strains of Salmonella enterica. This field was recently brought to the forefront of the scientific community and public interest by outbreaks of human illness linked to the consumption of fresh fruits and vegetables. Students were encouraged to develop their own testable hypotheses to compare proliferation of Salmonella enterica sv Typhimurium LT2 in different vegetables: cherry and regular-size tomatoes, onions, lettuce, and yellow and red bell peppers (Escherichia coli can be substituted for BSL1 laboratories). Upon completion of the laboratory experience, students were able to: 1) Develop testable hypotheses addressing the ability of a human pathogen, Salmonella enterica, to colonize and proliferate in vegetables; 2) Determine that different vegetables support the growth of Salmonella to different extents; 3) Conduct statistical analysis and identify any significant differences. The teaching-learning process was assessed with a pre-/posttest, with an average increase in content understanding from ~15% to 85%. We also measured students' proficiency while conducting specific technical tasks, revealing no major difficulties while conducting the experiments. Students indicated satisfaction with the organization and content of the practices. All of the students (100%) agreed that the exercises improved their knowledge of this subject.

Plant innate immunity against human bacterial pathogens

Certain human bacterial pathogens such as the enterohemorrhagic Escherichia coli and Salmonella enterica are not proven to be plant pathogens yet. Nonetheless, under certain conditions they can survive on, penetrate into, and colonize internal plant tissues causing serious food borne disease outbreaks. In this review, we highlight current understanding on the molecular mechanisms of plant responses against human bacterial pathogens and discuss salient common and contrasting themes of plant interactions with phytopathogens or human pathogens.