Salmonella enterica flagellin is recognized via FLS2 and activates PAMP-triggered immunity in Arabidopsis thaliana

From field to plate: How do bacterial enteric pathogens interact with ready-to-eat fruit and vegetables, causing disease outbreaks?

Ready-to-eat fruit and vegetables are a convenient source of nutrients and fibre for consumers, and are generally safe to eat, but are vulnerable to contamination with human enteric bacterial pathogens. Over the last decade, Salmonella spp., pathogenic Escherichia coli, and Listeria monocytogenes have been linked to most of the bacterial outbreaks of foodborne illness associated with fresh produce. The origins of these outbreaks have been traced to multiple sources of contamination from pre-harvest (soil, seeds, irrigation water, domestic and wild animal faecal matter) or post-harvest operations (storage, preparation and packaging). These pathogens have developed multiple processes for successful attachment, survival and colonization conferring them the ability to adapt to multiple environments. However, these processes differ across bacterial strains from the same species, and across different plant species or cultivars. In a competitive environment, additional risk factors are the plant microbiome phyllosphere and the plant responses; both factors directly modulate the survival of the pathogens on the leaf's surface. Understanding the mechanisms involved in bacterial attachment to, colonization of, and proliferation, on fresh produce and the role of the plant in resisting bacterial contamination is therefore crucial to reducing future outbreaks.

Bidirectional Comparisons Revealed Functional Patterns in Interaction between Salmonella enterica and Plants

Plants may harbor the human pathogen Salmonella enterica. Interactions between S. enterica and different plant species have been studied in individual reports. However, disparities arising from the distinct experimental conditions may render a meaningful comparison very difficult. This study explored interaction patterns between different S. enterica strains including serovars Typhimurium 14028s and LT2 and serovar Senftenberg, and different plants (Arabidopsis, lettuce, and tomato) in one approach. Better persistence of S. enterica serovar Typhimurium strains was observed in all tested plants, whereas the resulting symptoms varied depending on plant species. Genes encoding pathogenesis-related proteins were upregulated in plants inoculated with Salmonella. Furthermore, transcriptome of tomato indicated dynamic responses to Salmonella, with strong and specific responses already 24 h after inoculation. By comparing with publicly accessible Arabidopsis and lettuce transcriptome results generated in a similar manner, constants and variables were displayed. Plants responded to Salmonella with metabolic and physiological adjustments, albeit with variability in reprogrammed orthologues. At the same time, Salmonella adapted to plant leaf-mimicking media with changes in biosynthesis of cellular components and adjusted metabolism. This study provides insights into the Salmonella-plant interaction, allowing for a direct comparison of responses and adaptations in both organisms.

Intraspecies competition among Salmonella enterica isolates in the lettuce leaf apoplast

Multiple Salmonella enterica serovars and strains have been reported to be able to persist inside the foliar tissue of lettuce (Lactuca sativa L.), potentially resisting washing steps and reaching the consumer. Intraspecies variation of the bacterial pathogen and of the plant host can both significantly affect the outcome of foliar colonization. However, current understanding of the mechanisms underlying this phenomenon is still very limited. In this study, we evaluated the foliar fitness of 14 genetically barcoded S. enterica isolates from 10 different serovars, collected from plant and animal sources. The S. enterica isolates were vacuum-infiltrated individually or in pools into the leaves of three- to four-week-old lettuce plants. To estimate the survival capacity of individual isolates, we enumerated the bacterial populations at 0- and 10- days post-inoculation (DPI) and calculated their net growth. The competition of isolates in the lettuce apoplast was assessed through the determination of the relative abundance change of barcode counts of each isolate within pools during the 10 DPI experimental period. Isolates exhibiting varying apoplast fitness phenotypes were used to evaluate their capacity to grow in metabolites extracted from the lettuce apoplast and to elicit the reactive oxygen species burst immune response. Our study revealed that strains of S. enterica can substantially differ in their ability to survive and compete in a co-inhabited lettuce leaf apoplast. The differential foliar fitness observed among these S. enterica isolates might be explained, in part, by their ability to utilize nutrients available in the apoplast and to evade plant immune responses in this niche.

Salmonella enterica relies on carbon metabolism to adapt to agricultural environments

Salmonella enterica, a foodborne and human pathogen, is a constant threat to human health. Agricultural environments, for example, soil and plants, can be ecological niches and vectors for Salmonella transmission. Salmonella persistence in such environments increases the risk for consumers. Therefore, it is necessary to investigate the mechanisms used by Salmonella to adapt to agricultural environments. We assessed the adaptation strategy of S. enterica serovar Typhimurium strain 14028s to agricultural-relevant situations by analyzing the abundance of intermediates in glycolysis and the tricarboxylic acid pathway in tested environments (diluvial sand soil suspension and leaf-based media from tomato and lettuce), as well as in bacterial cells grown in such conditions. By reanalyzing the transcriptome data of Salmonella grown in those environments and using an independent RT-qPCR approach for verification, several genes were identified as important for persistence in root or leaf tissues, including the pyruvate dehydrogenase subunit E1 encoding gene aceE. In vivo persistence assay in tomato leaves confirmed the crucial role of aceE. A mutant in another tomato leaf persistence-related gene, aceB, encoding malate synthase A, displayed opposite persistence features. By comparing the metabolites and gene expression of the wild-type strain and its aceB mutant, fumarate accumulation was discovered as a potential way to replenish the effects of the aceB mutation. Our research interprets the mechanism of S. enterica adaptation to agriculture by adapting its carbon metabolism to the carbon sources available in the environment. These insights may assist in the development of strategies aimed at diminishing Salmonella persistence in food production systems.

Microgravity and evasion of plant innate immunity by human bacterial pathogens

Spaceflight microgravity and modeled-microgravity analogs (MMA) broadly alter gene expression and physiology in both pathogens and plants. Research elucidating plant and bacterial responses to normal gravity or microgravity has shown the involvement of both physiological and molecular mechanisms. Under true and simulated microgravity, plants display differential expression of pathogen-defense genes while human bacterial pathogens exhibit increased virulence, antibiotic resistance, stress tolerance, and reduced LD50 in animal hosts. Human bacterial pathogens including Salmonella enterica and E. coli act as cross-kingdom foodborne pathogens by evading and suppressing the innate immunity of plants for colonization of intracellular spaces. It is unknown if evasion and colonization of plants by human pathogens occurs under microgravity and if there is increased infection capability as demonstrated using animal hosts. Understanding the relationship between microgravity, plant immunity, and human pathogens could prevent potentially deadly outbreaks of foodborne disease during spaceflight. This review will summarize (1) alterations to the virulency of human pathogens under microgravity and MMA, (2) alterations to plant physiology and gene expression under microgravity and MMA, (3) suppression and evasion of plant immunity by human pathogens under normal gravity, (4) studies of plant-microbe interactions under microgravity and MMA. A conclusion suggests future study of interactions between plants and human pathogens under microgravity is beneficial to human safety, and an investment in humanity's long and short-term space travel goals.

The effect of ε-poly-l-lysine treatment on molecular, physiological and biochemical indicators related to resistance in longan fruit infected by Phomopsis longanae Chi

Postharvest longan fruits are subjected to Phomopsis longanae Chi (P. longanae) infection that lead to fruit quality deterioration. We hypothesized that ε-poly-l-lysine (ε-PL) could enhance fruit disease resistance in longans. Through physiological and transcriptomic analyses, the results showed that, compared to P. longanae-infected longan fruit, ε-PL + P. longanae treatment reduced the disease development of longan fruits. Additionally, ε-PL + P. longanae treatment increased the contents of disease-resistant substances (lignin and H₂O₂) and the activities of disease-resistance enzymes (CHI, PAL, PPO, C4H, CAD, GLU, 4CL, and POD). Furthermore, the expressions of genes relevant to the phenylpropanoid biosynthesis pathway and plant-pathogen interaction pathway (Rboh, FLS2, WRKY29, FRK1, and PR1) were up-regulated by ε-PL + P. longanae treatment. These findings demonstrated that ε-PL treatment inhibited the disease development of postharvest longan fruits were associated with the increased accumulation of disease-resistant related substances, as well as the raised activities and genes expressions of disease-resistance related enzymes.

Fresh Produce as a Potential Vector and Reservoir for Human Bacterial Pathogens: Revealing the Ambiguity of Interaction and Transmission

The consumer demand for fresh produce (vegetables and fruits) has considerably increased since the 1980s for more nutritious foods and healthier life practices, particularly in developed countries. Currently, several foodborne outbreaks have been linked to fresh produce. The global rise in fresh produce associated with human infections may be due to the use of wastewater or any contaminated water for the cultivation of fruits and vegetables, the firm attachment of the foodborne pathogens on the plant surface, and the internalization of these agents deep inside the tissue of the plant, poor disinfection practices and human consumption of raw fresh produce. Several investigations have been established related to the human microbial pathogens (HMPs) interaction, their internalization, and survival on/within plant tissue. Previous studies have displayed that HMPs are comprised of several cellular constituents to attach and adapt to the plant’s intracellular niches. In addition, there are several plant-associated factors, such as surface morphology, nutrient content, and plant–HMP interactions, that determine the internalization and subsequent transmission to humans. Based on documented findings, the internalized HMPs are not susceptible to sanitation or decontaminants applied on the surface of the fresh produce. Therefore, the contamination of fresh produce by HMPs could pose significant food safety hazards. This review provides a comprehensive overview of the interaction between fresh produce and HMPs and reveals the ambiguity of interaction and transmission of the agents to humans.

RpoS contributes in a host-dependent manner to Salmonella colonization of the leaf apoplast during plant disease

Contaminated fresh produce has been routinely linked to outbreaks of Salmonellosis. Multiple studies have identified Salmonella enterica factors associated with successful colonization of diverse plant niches and tissues. It has also been well documented that S. enterica can benefit from the conditions generated during plant disease by host-compatible plant pathogens. In this study, we compared the capacity of two common S. enterica research strains, 14028s and LT2 (strain DM10000) to opportunistically colonize the leaf apoplast of two model plant hosts Arabidopsis thaliana and Nicotiana benthamiana during disease. While S. enterica 14028s benefited from co-colonization with plant-pathogenic Pseudomonas syringae in both plant hosts, S. enterica LT2 was unable to benefit from Pto co-colonization in N. benthamiana. Counterintuitively, LT2 grew more rapidly in ex planta N. benthamiana apoplastic wash fluid with a distinctly pronounced biphasic growth curve in comparison with 14028s. Using allelic exchange, we demonstrated that both the N. benthamiana infection-depedent colonization and apoplastic wash fluid growth phenotypes of LT2 were associated with mutations in the S. enterica rpoS stress-response sigma factor gene. Mutations of S. enterica rpoS have been previously shown to decrease tolerance to oxidative stress and alter metabolic regulation. We identified rpoS-dependent alterations in the utilization of L-malic acid, an abundant carbon source in N. benthamiana apoplastic wash fluid. We also present data consistent with higher relative basal reactive oxygen species (ROS) in N. benthamiana leaves than in A. thaliana leaves. The differences in basal ROS may explain the host-dependent disease co-colonization defect of the rpoS-mutated LT2 strain. Our results indicate that the conducive environment generated by pathogen modulation of the apoplast niche can vary from hosts to host even with a common disease-compatible pathogen.

Xanthomonas Infection Transforms the Apoplast into an Accessible and Habitable Niche for Salmonella enterica

Bacterial spot disease caused by Xanthomonas species devastates tomato production worldwide. Salmonellosis outbreaks from consumption of raw produce have been linked to the arrival of Salmonella enterica on crop plants in the field via contaminated irrigation water.

Vibrio splendidus flagellin C binds tropomodulin to induce p38 MAPK-mediated p53-dependent coelomocyte apoptosis in Echinodermata

As a typical pathogen-associated molecular pattern, bacterial flagellin can bind Toll-like receptor 5 and the intracellular NAIP5 receptor component of the NLRC4 inflammasome to induce immune responses in mammals. However, these flagellin receptors are generally poorly understood in lower animal species. In this study, we found that the isolated flagellum of Vibrio splendidus AJ01 destroyed the integrity of the tissue structure of coelomocytes and promoted apoptosis in the sea cucumber Apostichopus japonicus. To further investigate the molecular mechanism, the novel intracellular LRR domain-containing protein tropomodulin (AjTmod) was identified as a protein that interacts with flagellin C (FliC) with a dissociation constant (K_d) of 0.0086 ± 0.33 μM by microscale thermophoresis assay. We show that knockdown of AjTmod also depressed FliC-induced apoptosis of coelomocytes. Further functional analysis with different inhibitor treatments revealed that the interaction between AjTmod and FliC could specifically activate p38 MAPK, but not JNK or ERK MAP kinases. We demonstrate that the transcription factor p38 is then translocated into the nucleus, where it mediates the expression of p53 to induce coelomocyte apoptosis. Our findings provide the first evidence that intracellular AjTmod serves as a novel receptor of FliC and mediates p53-dependent coelomocyte apoptosis by activating the p38 MAPK signaling pathway in Echinodermata.

FLS2–RBOHD–PIF4 Module Regulates Plant Response to Drought and Salt Stress

As sessile organisms, plants are constantly challenged by several environmental stresses. Different kinds of stress often occur simultaneously, leading to the accumulation of reactive oxygen species (ROS) produced by respiratory burst oxidase homolog (RBOHD) and calcium fluctuation in cells. Extensive studies have revealed that flagellin sensitive 2 (FLS2) can sense the infection by pathogenic microorganisms and activate cellular immune response by regulating intracellular ROS and calcium signals, which can also be activated during plant response to abiotic stress. However, little is known about the roles of FLS2 and RBOHD in regulating abiotic stress. In this study, we found that although the fls2 mutant showed tolerance, the double mutant rbohd rbohf displayed hypersensitivity to abiotic stress, similar to its performance in response to immune stress. An analysis of the transcriptome of the fls2 mutant and rbohd rbohf double mutant revealed that phytochrome interacting factor 4 (PIF4) acted downstream of FLS2 and RBOHD to respond to the abiotic stress. Further analysis showed that both FLS2 and RBOHD regulated the response of plants to drought and salt stress by regulating the expression of PIF4. These findings revealed an FLS2–RBOHD–PIF4 module in regulating plant response to biotic and abiotic stresses.

Dual transcriptomic analysis reveals metabolic changes associated with differential persistence of human pathogenic bacteria in leaves of Arabidopsis and lettuce

Understanding the molecular determinants underlying the interaction between the leaf and human pathogenic bacteria is key to provide the foundation to develop science-based strategies to prevent or decrease the pathogen contamination of leafy greens. In this study, we conducted a dual RNA-sequencing analysis to simultaneously define changes in the transcriptomic profiles of the plant and the bacterium when they come in contact. We used an economically relevant vegetable crop, lettuce (Lactuca sativa L. cultivar Salinas), and a model plant, Arabidopsis thaliana Col-0, as well as two pathogenic bacterial strains that cause disease outbreaks associated with fresh produce, Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium 14028s (STm 14028s). We observed commonalities and specificities in the modulation of biological processes between Arabidopsis and lettuce and between O157:H7 and STm 14028s during early stages of the interaction. We detected a larger alteration of gene expression at the whole transcriptome level in lettuce and Arabidopsis at 24 h post inoculation with STm 14028s compared to that with O157:H7. In addition, bacterial transcriptomic adjustments were substantially larger in Arabidopsis than in lettuce. Bacterial transcriptome was affected at a larger extent in the first 4 h compared to the subsequent 20 h after inoculation. Overall, we gained valuable knowledge about the responses and counter-responses of both bacterial pathogen and plant host when these bacteria are residing in the leaf intercellular space. These findings and the public genomic resources generated in this study are valuable for additional data mining.

Effect of Plant Systemic Resistance Elicited by Biological and Chemical Inducers on the Colonization of the Lettuce and Basil Leaf Apoplast by Salmonella enterica

Mitigation strategies to prevent microbial contamination of crops are lacking. We tested the hypothesis that induction of plant systemic resistance by biological (induced systemic resistance [ISR]) and chemical (systemic acquired resistance [SAR]) elicitors reduces endophytic colonization of leaves by Salmonella enterica serovars Senftenberg and Typhimurium. S. Senftenberg had greater endophytic fitness than S. Typhimurium in basil and lettuce. The apoplastic population sizes of serovars Senftenberg and Typhimurium in basil and lettuce, respectively, were significantly reduced approximately 10- to 100-fold by root treatment with microbial inducers of systemic resistance compared to H2O treatment. Rhodotorula glutinis effected the lowest population increases of S. Typhimurium in lettuce and S. Senftenberg in basil leaves, respectively 120- and 60-fold lower than those seen with the H2O treatment over 10 days postinoculation. Trichoderma harzianum and Pichia guilliermondii did not have any significant effect on S. Senftenberg in the basil apoplast. The chemical elicitors acidobenzolar-S-methyl and dl-β-amino-butyric acid inhibited S. Typhimurium multiplication in the lettuce apoplast 10- and 2-fold, respectively, compared to H2O-treated plants. All ISR and SAR inducers applied to lettuce roots in this study increased leaf expression of the defense gene PR1, as did Salmonella apoplastic colonization in H2O-treated lettuce plants. Remarkably, both acidobenzolar-S-methyl upregulation and R. glutinis upregulation of PR1 were repressed by the presence of Salmonella in the leaves. However, enhanced PR1 expression was sustained longer and at greater levels upon elicitor treatment than by Salmonella induction alone. These results serve as a proof of concept that priming of plant immunity may provide an intrinsic hurdle against the endophytic establishment of enteric pathogens in leafy vegetables. IMPORTANCE Fruit and vegetables consumed raw have become an important vehicle of foodborne illness despite a continuous effort to improve their microbial safety. Salmonella enterica has caused numerous recalls and outbreaks of infection associated with contaminated leafy vegetables. Evidence is increasing that enteric pathogens can reach the leaf apoplast, where they confront plant innate immunity. Plants may be triggered for induction of their defense signaling pathways by exposure to chemical or microbial elicitors. This priming for recognition of microbes by plant defense pathways has been used to inhibit plant pathogens and limit disease. Given that current mitigation strategies are insufficient in preventing microbial contamination of produce and associated outbreaks, we investigated the effect of plant-induced resistance on S. enterica colonization of the lettuce and basil leaf apoplast in order to gain a proof of concept for the use of such an intrinsic approach to inhibit human pathogens in leafy vegetables.

Plants as a realized niche for Listeria monocytogenes

Listeria monocytogenes is a human pathogen. It is the causative agent of listeriosis, the leading cause of bacterial-linked foodborne mortality in Europe and elsewhere. Outbreaks of listeriosis have been associated with the consumption of fresh produce including vegetables and fruits. In this review we summarize current data providing direct or indirect evidence that plants can serve as habitat for L. monocytogenes, enabling this human pathogen to survive and grow. The current knowledge of the mechanisms involved in the interaction of this bacterium with plants is addressed, and whether this foodborne pathogen elicits an immune response in plants is discussed.

ARABIDOPSIS MUESTRA RESISTENCIA NO-HOSPEDERO CONSTITUTIVA CONTRA Xanthomonas phaseoli pv. manihotis

La bacteriosis vascular de la yuca, causada por la bacteria gram negativa Xanthomonas phaseoli pv. manihotis (Xpm), anteriormente conocida como Xanthomonas axonopodis pv. manihotis, es la principal enfermedad bacteriana que compromete su producción. Con la meta de generar una resistencia durable y de amplio espectro a la bacteriosis es posible explotar los mecanismos naturales presentes en plantas no-hospedero. Arabidopsis es una planta modelo extensamente estudiada, la cual es no-hospedero de Xpm. La meta de este estudio fue determinar si la resistencia no-hospedero de Arabidopsis es consecuencia de la presencia de barreras físicas o si esta depende de determinantes genéticos. En este trabajo se evaluó la capacidad de plantas de Arabidopsis de responder a la inoculación con Xpm. Ninguno de los ocho ecotipos de Arabidopsis evaluados mostraron una respuesta hipersensible a la inoculación con ocho diferentes cepas de Xpm. Aunque no se identificó la presencia de especies reactivas de oxígeno si se encontró un bloqueo en el crecimiento de Xpm en las plantas de Arabidopsis. En conjunto, los resultados aquí presentados sugieren que Arabidopsis no está activando una respuesta contra Xpm y que la resistencia observada puede ser consecuencia de las barreras físicas presentes en Arabidopsis que Xpm no es capaz de superar.

Mechanisms adopted by Salmonella to colonize plant hosts

Fruits and vegetables consumed fresh or as minimally-processed produce, have multiple benefits for our diet. Unfortunately, they bring a risk of food-borne diseases, for example salmonellosis. Interactions between Salmonella and crop plants are indeed a raising concern for the global health. Salmonella uses multiple strategies to manipulate the host defense system, including plant's defense responses. The main focus of this review are strategies used by this bacterium during the interaction with crop plants. Emphasis was put on how Salmonella avoids the plant defense responses and successfully colonizes plants. In addition, several factors were reviewed assessing their impact on Salmonella persistence and physiological adaptation to plants and plant-related environment. The understanding of those mechanisms, their regulation and use by the pathogen, while in contact with plants, has significant implication on the growth, harvest and processing steps in plant production system. Consequently, it requires both the authorities and science to advance and definite methods aiming at prevention of crop plants contamination. Thus, minimizing and/or eliminating the potential of human diseases.

Chimeric Tobamoviruses With Coat Protein Exchanges Modulate Symptom Expression and Defence Responses in Nicotiana tabacum

In the pathogen infection and host defence equilibrium, plant viruses have evolved to efficiently replicate their genomes, to resist the attack from host defence responses and to avoid causing severe negative effect on growth and metabolism of the hosts. In this study, we generated chimeric tobacco mosaic virus (TMV) variants, in which the coat protein (CP) sequences were substituted with that of cucumber green mottle mosaic virus (CGMMV) or pepper mild mottle virus (PMMoV) to address the role of these in virus infection and host symptomology. The results showed that the chimeric viruses (TMV-CGCP or TMV-PMCP) induce stunting and necrotic symptoms in tobacco plants. We analyzed the transcriptomic changes in tobacco plants after infection of TMV and its chimeras using a high-throughput RNA sequencing approach and found that infection of the chimeric TMV induced significant up-regulation of host defence responsive genes together with salicylic (SA) or abscisic acid (ABA) responsive genes, but down-regulation of auxin (Aux) responsive genes. We further confirmed the increase in the levels of SA and ABA, together with the reduced levels of Aux after infection of chimeric TMV in tobacco plants. These data suggest novel roles of tobamovirus CP in induction of host symptoms and defence responses.

Transcriptomic and functional analyses reveal an antiviral role of autophagy during pepper mild mottle virus infection

BACKGROUND

Pepper mild mottle virus (PMMoV) is a member in the genus Tobamovirus and infects mainly solanaceous plants. However, the mechanism of virus-host interactions remains unclear. To explore the responses of pepper plants to PMMoV infection, we analyzed the transcriptomic changes in pepper plants after PMMoV infection using a high-throughput RNA sequencing approach and explored the roles of host autophagy in regulating PMMoV infection.

RESULTS

A total of 197 differentially expressed genes (DEGs) were obtained after PMMoV infection, including 172 significantly up-regulated genes and 25 down-regulated genes. The Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses revealed that most up-regulated DEGs were involved in plant abiotic and biotic stresses. Further analyses showed the expressions of multiple autophagy-related genes (ATGs) were increased after PMMoV infection in pepper and Nicotiana benthamiana plants. Through confocal microscopy and transmission electron microscopy, we have found that PMMoV infection in plant can induce autophagy, evidenced by the increased number of GFP-ATG8a fluorescent punctate and the appearance of double membrane autophagic structures in cells of N. benthamiana. Additionally, inhibition of autophagy significantly increased PMMoV RNA accumulation and aggravated systemic PMMoV symptoms through autophagy inhibitor (3-MA and E64d) treatment and silencing of NbATG expressions by a Tobacco rattle virus-induced gene silencing assays. These results indicated that autophagy played a positive role in plant resistance to PMMoV infection.

CONCLUSIONS

Taken together, our results provide a transcriptomic insight into pepper responding to PMMoV infection and reveal that autophagy induced by PMMoV infection has an antiviral role in regulating PMMoV infection. These results also help us to better understand the mechanism controlling PMMoV infection in plants and to develop better strategies for breeding projects for virus-resistant crops.

Stress response, amino acid biosynthesis and pathogenesis genes expressed in Salmonella enterica colonizing tomato shoot and root surfaces

Salmonella enterica can colonize all parts of the tomato plant. Tomatoes have been frequently implicated in salmonellosis outbreaks. In agricultural settings, Salmonella must overcome stress, nutritional and competition barriers to become established on plant surfaces. Knowledge of the genetic mechanisms underlying Salmonella-plant associations is limited, especially when growing epiphytically. A genome-wide transcriptomic analysis of Salmonella Typhimurium (SeT) was conducted with RNA-Seq to elucidate strategies for epiphytic growth on live, intact tomato shoot and root surfaces. Six plasmid-encoded and 123 chromosomal genes were significantly (using Benjamini-Hochberg adjusted p-values) up-regulated; 54 and 110 detected in SeT on shoots and roots, respectively, with 35 common to both. Key signals included NsrR regulon genes needed to mitigate nitrosative stress, oxidative stress genes and host adaptation genes, including environmental stress, heat shock and acid-inducible genes. Several amino acid biosynthesis genes and genes indicative of sulphur metabolism and anaerobic respiration were up-regulated. Some Type III secretion system (T3SS) effector protein genes and their chaperones from pathogenicity island-2 were expressed mostly in SeT on roots. Gene expression in SeT was validated against SeT and also the tomato outbreak strain Salmonella Newport with a high correlation (R ² = 0.813 and 0.874, respectively; both p < 0.001). Oxidative and nitrosative stress response genes, T3SS2 genes and amino acid biosynthesis may be needed for Salmonella to successfully colonize tomato shoot and root surfaces.

Salmonella Heterogeneously Expresses Flagellin during Colonization of Plants

Minimally processed or fresh fruits and vegetables are unfortunately linked to an increasing number of food-borne diseases, such as salmonellosis. One of the relevant virulence factors during the initial phases of the infection process is the bacterial flagellum. Although its function is well studied in animal systems, contradictory results have been published regarding its role during plant colonization. In this study, we tested the hypothesis that Salmonella's flagellin plays a versatile function during the colonization of tomato plants. We have assessed the persistence in plant tissues of a Salmonella enterica wild type strain, and of a strain lacking the two flagellins, FljB and FliC. We detected no differences between these strains concerning their respective abilities to reach distal, non-inoculated parts of the plant. Analysis of flagellin expression inside the plant, at both the population and single cell levels, shows that the majority of bacteria down-regulate flagellin production, however, a small fraction of the population continues to express flagellin at a very high level inside the plant. This heterogeneous expression of flagellin might be an adaptive strategy to the plant environment. In summary, our study provides new insights on Salmonella adaption to the plant environment through the regulation of flagellin expression.

Evasion of Plant Innate Defense Response by Salmonella on Lettuce

To establish host association, the innate immune system, which is one of the first lines of defense against infectious disease, must be circumvented. Plants encounter enteric foodborne bacterial pathogens under both pre- and post-harvest conditions. Human enteric foodborne pathogens can use plants as temporary hosts. This unique interaction may result in recalls and illness outbreaks associated with raw agricultural commodities. The purpose of this study was to determine if Salmonella enterica Typhimurium applied to lettuce leaves can suppress the innate stomatal defense in lettuce and utilization of UD1022 as a biocontrol against this ingression. Lettuce leaves were spot inoculated with S. Typhimurium wild type and its mutants. Bacterial culture and confocal microscopy analysis of stomatal apertures were used to support findings of differences in S. Typhimurium mutants compared to wild type. The persistence and internalization of these strains on lettuce was compared over a 7-day trial. S. Typhimurium may bypass the innate stomatal closure defense response in lettuce. Interestingly, a few key T3SS components in S. Typhimurium were involved in overriding stomatal defense response in lettuce for ingression. We also show that the T3SS in S. Typhimurium plays a critical role in persistence of S. Typhimurium in planta. Salmonella populations were significantly reduced in all UD1022 groups by day 7 with the exception of fliB and invA mutants. Salmonella internalization was not detected in plants after UD1022 treatment and had significantly higher stomatal closure rates (aperture width = 2.34 μm) by day 1 compared to controls (8.5 μm). S. Typhimurium SPI1 and SPI2 mutants showed inability to reopen stomates in lettuce suggesting the involvement of key T3SS components in suppression of innate response in plants. These findings impact issues of contamination related to plant performance and innate defense responses for plants.

Salmonella enterica Elicits and Is Restricted by Nitric Oxide and Reactive Oxygen Species on Tomato

The enteric pathogen Salmonella enterica can interact with parts of the plant immune system despite not being a phytopathogen. Previous transcriptomic profiling of S. enterica associating with tomato suggested that Salmonella was responding to oxidative and nitrosative stress in the plant niche. We aimed to investigate whether Salmonella was eliciting generation of reactive oxygen species (ROS) and nitric oxide (NO), two components of the microbe-associated molecular pattern (MAMP)-triggered immunity (MTI) of plants. We also sought to determine whether this interaction had any measurable effects on Salmonella colonization of plants. Biochemical, gene expression and on-plant challenge assays of tomato vegetative and fruit organs were conducted to assess the elicitation of ROS and NO in response to Salmonella Newport association. The counter bacterial response and the effect of NO and ROS on Salmonella colonization was also investigated. We detected H₂O₂ in leaves and fruit following challenge with live S. Newport (p < 0.05). Conversely, NO was detected on leaves but not on fruit in response to S. Newport (p < 0.05). We found no evidence of plant defense attenuation by live S. Newport. Bacterial gene expression of S. Newport associating with leaves and fruit were indicative of adaptation to biotic stress in the plant niche. The nitrosative stress response genes hmpA and yoaG were significantly up-regulated in S. Newport on leaves and fruit tissue compared to tissue scavenged of NO or ROS (p < 0.05). Chemical modulation of these molecules in the plant had a restrictive effect on bacterial populations. Significantly higher S. Newport titers were retrieved from H₂O₂ scavenged leaves and fruit surfaces compared to controls (p < 0.05). Similarly, S. Newport counts recovered from NO-scavenged leaves, but not fruit, were higher compared to control (p < 0.05), and significantly lower on leaves pre-elicited to produce endogenous NO. We present evidence of Salmonella elicitation of ROS and NO in tomato, which appear to have a restricting effect on the pathogen. Moreover, bacterial recognition of ROS and NO stress was detected. This work shows that tomato has mechanisms to restrict Salmonella populations and ROS and NO detoxification may play an important role in Salmonella adaptation to the plant niche.

Factors Required for Adhesion of Salmonella enterica Serovar Typhimurium to Corn Salad (Valerianella locusta)

Salmonella enterica is a foodborne pathogen often leading to gastroenteritis and is commonly acquired by consumption of contaminated food of animal origin. However, frequency of outbreaks linked to the consumption of fresh or minimally processed food of nonanimal origin is increasing. New infection routes of S. enterica by vegetables, fruits, nuts, and herbs have to be considered. This leads to special interest in S. enterica interactions with leafy products, e.g., salads, that are mainly consumed in a minimally processed form. The attachment of S. enterica to salad is a crucial step in contamination, but little is known about the bacterial factors required and mechanisms of adhesion. S. enterica possesses a complex set of adhesive structures whose functions are only partly understood. Potentially, S. enterica may deploy multiple adhesive strategies for adhering to various salad species and other vegetables. In this study, we systematically analyzed the contributions of the complete adhesiome, of lipopolysaccharide (LPS), and of flagellum-mediated motility of S. enterica serovar Typhimurium (STM) in adhesion to Valerianella locusta (corn salad). We deployed a reductionist, synthetic approach to identify factors involved in the surface binding of STM to leaves of corn salad, with particular regard to the expression of all known adhesive structures, using the Tet-on system. This work reveals the contribution of Saf fimbriae, type 1 secretion system-secreted BapA, an intact LPS, and flagellum-mediated motility of STM in adhesion to corn salad leaves.IMPORTANCE Transmission of gastrointestinal pathogens by contaminated fresh produce is of increasing relevance to human health. However, the mechanisms of contamination of, persistence on, and transmission by fresh produce are poorly understood. We investigated the contributions of the various adhesive structures of STM to the initial event in transmission, i.e., binding to the plant surface. A reductionist system was used that allowed experimentally controlled surface expression of individual adhesive structures and analyses of the contribution to binding to leave surfaces of corn salad under laboratory conditions. The model system allowed the determination of the relative contributions of fimbrial and nonfimbrial adhesins, the type 3 secretion systems, the O antigen of lipopolysaccharide, the flagella, and chemotaxis of STM to binding to corn salad leaves. Based on these data, future work could reveal the mechanism of binding and the relevance of interaction under agricultural conditions.

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.

Novel molecular components involved in callose-mediated Arabidopsis defense against Salmonella enterica and Escherichia coli O157:H7

BACKGROUND

Food contamination with Salmonella enterica and enterohemorrhagic Escherichia coli is among the leading causes of foodborne illnesses worldwide and crop plants are associated with > 50% of the disease outbreaks. However, the mechanisms underlying the interaction of these human pathogens with plants remain elusive. In this study, we have explored plant resistance mechanisms against these enterobacteria and the plant pathogen Pseudomonas syringae pv. tomato (Pst) DC3118, as an opportunity to improve food safety.

RESULTS

We found that S. enterica serovar Typhimurium (STm) transcriptionally modulates stress responses in Arabidopsis leaves, including induction of two hallmark processes of plant defense: ROS burst and cell wall modifications. Analyses of plants with a mutation in the potentially STm-induced gene EXO70H4 revealed that its encoded protein is required for stomatal defense against STm and E. coli O157:H7, but not against Pst DC3118. In the apoplast however, EXO70H4 is required for defense against STm and Pst DC3118, but not against E. coli O157:H7. Moreover, EXO70H4 is required for callose deposition, but had no function in ROS burst, triggered by all three bacteria. The salicylic acid (SA) signaling and biosynthesis proteins NPR1 and ICS1, respectively, were involved in stomatal and apoplastic defense, as well as callose deposition, against human and plant pathogens.

CONCLUSIONS

The results show that EXO70H4 is involved in stomatal and apoplastic defenses in Arabidopsis and suggest that EXO70H4-mediated defense play a distinct role in guard cells and leaf mesophyll cells in a bacteria-dependent manner. Nonetheless, EXO70H4 contributes to callose deposition in response to both human and plant pathogens. NPR1 and ICS1, two proteins involved in the SA signaling pathway, are important to inhibit leaf internalization and apoplastic persistence of enterobacteria and proliferation of phytopathogens. These findings highlight the existence of unique and shared plant genetic components to fight off diverse bacterial pathogens providing specific targets for the prevention of foodborne diseases.

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 enterica serovar Typhimurium ATCC 14028S is tolerant to plant defenses triggered by the flagellin receptor FLS2

Salmonellosis outbreaks associated with sprouted legumes have been a food safety concern for over two decades. Despite evidence that Salmonella enterica triggers biotic plant defense pathways, it has remained unclear how plant defenses impact Salmonella growth on sprouted legumes. We used Medicago truncatula mutants in which the gene for the flagellin receptor FLS2 was disrupted to demonstrate that plant defenses triggered by FLS2 elicitation do not impact the growth of Salmonella enterica serovar Typhimurium ATCC 14028S. As a control, we tested the growth of Salmonella enterica serovar Typhimurium LT2, which has a defect in rpoS that increases its sensitivity to reactive oxygen species. LT2 displayed enhanced growth on M. truncatula FLS2 mutants in comparison to wild-type M. truncatula. We hypothesize that these growth differences are primarily due to differences in 14028S and LT2 reactive oxygen species sensitivity. Results from this study show that FLS2-mediated plant defenses are ineffective in inhibiting growth of Salmonella entrica 14028S.

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.

Molecular Regulation of Host Defense Responses Mediated by Biological Anti-TMV Agent Ningnanmycin

Ningnanmycin (NNM) belongs to microbial pesticides that display comprehensive antiviral activity against plant viruses. NNM treatment has been shown to efficiently delay or suppress the disease symptoms caused by tobacco mosaic virus (TMV) infection in local-inoculated or systemic-uninoculated tobacco leaves, respectively. However, the underlying molecular mechanism of NNM-mediated antiviral activity remains to be further elucidated. In this study, 414 differentially expressed genes (DEGs), including 383 which were up-regulated and 31 down-regulated, caused by NNM treatment in TMV-infected BY-2 protoplasts, were discovered by RNA-seq. In addition, KEGG analysis indicated significant enrichment of DEGs in the plant-pathogen interaction and MAPK signaling pathway. The up-regulated expression of crucial DEGs, including defense-responsive genes, such as the receptor-like kinase FLS2, RLK1, and the mitogen-activated protein kinase kinase kinase MAPKKK, calcium signaling genes, such as the calcium-binding protein CML19, as well as phytohormone responsive genes, such as the WRKY transcription factors WRKY40 and WRKY70, were confirmed by RT-qPCR. These findings provided valuable insights into the antiviral mechanisms of NNM, which indicated that the agent induces tobacco systemic resistance against TMV via activating multiple plant defense signaling pathways.

Green Technology: Bacteria-Based Approach Could Lead to Unsuspected Microbe⁻Plant⁻Animal Interactions

The recent and massive revival of green strategies to control plant diseases, mainly as a consequence of the Integrated Pest Management (IPM) rules issued in 2009 by the European Community and the increased consumer awareness of organic products, poses new challenges for human health and food security that need to be addressed in the near future. One of the most important green technologies is biocontrol. This approach is based on living organisms and how these biocontrol agents (BCAs) directly or indirectly interact as a community to control plant pathogens and pest. Although most BCAs have been isolated from plant microbiomes, they share some genomic features, virulence factors, and trans-kingdom infection abilities with human pathogenic microorganisms, thus, their potential impact on human health should be addressed. This evidence, in combination with the outbreaks of human infections associated with consumption of raw fruits and vegetables, opens new questions regarding the role of plants in the human pathogen infection cycle. Moreover, whether BCAs could alter the endophytic bacterial community, thereby leading to the development of new potential human pathogens, is still unclear. In this review, all these issues are debated, highlighting that the research on BCAs and their formulation should include these possible long-lasting consequences of their massive spread in the environment.

Root mediated uptake of Salmonella is different from phyto-pathogen and associated with the colonization of edible organs

BACKGROUND

Pre-harvest contamination of fruits and vegetables by Salmonella in fields is one of the causes of food-borne outbreaks. Natural openings like stomata, hydathodes and fruit cracks are known to serve as entry points. While there are reports indicating that Salmonella colonize and enter root through lateral root emerging area, further investigations regarding how the accessibility of Salmonella to lateral root is different from phyto-pathogenic bacteria, the efficacy of lateral root to facilitate entry have remained unexplored. In this study we attempted to investigate the lateral root mediated entry of Salmonella, and to bridge this gap in knowledge.

RESULTS

Unlike phytopathogens, Salmonella cannot utilize cellulose as the sole carbon source. This negates the fact of active entry by degrading plant cellulose and pectin. Endophytic Salmonella colonization showed a high correlation with number of lateral roots. When given equal opportunity to colonize the plants with high or low lateral roots, Salmonella internalization was found higher in the plants with more lateral roots. However, the epiphytic colonization in both these plants remained unaltered. To understand the ecological significance, we induced lateral root production by increasing soil salinity which made the plants susceptible to Salmonella invasion and the plants showed higher Salmonella burden in the aerial organs.

CONCLUSION

Salmonella, being unable to degrade plant cell wall material relies heavily on natural openings. Therefore, its invasion is highly dependent on the number of lateral roots which provides an entry point because of the epidermis remodeling. Thus, when number of lateral root was enhanced by increasing the soil salinity, plants became susceptible to Salmonella invasion in roots and its transmission to aerial organs.

Leafhopper-Induced Activation of the Jasmonic Acid Response Benefits Salmonella enterica in a Flagellum-Dependent Manner

Enteric human pathogens such as Salmonella enterica are typically studied in the context of their animal hosts, but it has become apparent that these bacteria spend a significant portion of their life cycle on plants. S. enterica survives the numerous stresses common to a plant niche, including defense responses, water and nutrient limitation, and exposure to UV irradiation leading to an increased potential for human disease. In fact, S. enterica is estimated to cause over one million cases of foodborne illness each year in the United States with 20% of those cases resulting from consumption of contaminated produce. Although S. enterica successfully persists in the plant environment, phytobacterial infection by Pectobacterium carotovorum or Xanthomonas spp. increases S. enterica survival and infrequently leads to growth on infected plants. The co-association of phytophagous insects, such as the Aster leafhopper, Macrosteles quadrilineatus, results in S. enterica populations that persist at higher levels for longer periods of time when compared to plants treated with S. enterica alone. We hypothesized that leafhoppers increase S. enterica persistence by altering the plant defense response to the benefit of the bacteria. Leafhopper infestation activated the jasmonic acid (JA) defense response while S. enterica colonization triggered the salicylic acid (SA) response. In tomato plants co-treated with S. enterica and leafhoppers, both JA- and SA-inducible genes were activated, suggesting that the presence of leafhoppers may affect the crosstalk that occurs between the two immune response pathways. To rule out the possibility that leafhoppers provide additional benefits to S. enterica, plants were treated with a chemical JA analog to activate the immune response in the absence of leafhoppers. Although bacterial populations continue to decline over time, analog treatment significantly increased bacterial persistence on the leaf surface. Bacterial mutant analysis determined that the bacterial flagellum, whether functional or not, was required for increased S. enterica survival after analog treatment. By investigating the interaction between this human pathogen, a common phytophagous insect, and their plant host, we hope to elucidate the mechanisms promoting S. enterica survival on plants and provide information to be used in the development of new food safety intervention strategies.

Microbial pollution and food safety

Microbial pollution is a serious food safety issue because it can lead to a wide range of foodborne diseases. A great number of foodborne diseases and outbreaks are reported in which contamination of fresh produce and animal products occurs from polluted sources with pathogenic bacteria, viruses and protozoa and such outbreaks are reviewed and the sources are revealed. Investigations of foodborne outbreaks involved meat production and fresh produce, namely, that occurred at the early stages of the food chain have shown certain sources of contamination. Domesticated food animals, as well as wild animals, flies and rodents can serve as a source of contamination of nearby produce-growing fields and can lead to human infection through direct contact at farms and, mostly, mail order hatcheries. The most of the fresh produce associated outbreaks have followed wildlife intrusion into growing fields or fecal contamination from nearly animal production facilities that likely led to produce contamination, polluted water used for irrigation and improper manure. Preventive measures, as part of implemented good agricultural practice systems are described. Controlling and minimizing pre-harvest contamination may be one of the key aspects of food safety.

Survival and interaction of Escherichia coli O104:H4 on Arabidopsis thaliana and lettuce (Lactuca sativa) in comparison to E. coli O157:H7: Influence of plant defense response and bacterial capsular polysaccharide

Shiga toxin-producing Escherichia coli (STEC) has been associated with illnesses and outbreaks linked to fresh vegetables, prompting a growing public health concern. Most studies regarding interactions of STEC on fresh produce focused on E. coli O157:H7. Limited information is available about survival or fitness of E. coli O104:H4, non-O157 pathogen that was linked to one of the largest outbreaks of hemolytic uremic syndrome in 2011. In this study, survival of E. coli O104:H4 was evaluated on Arabidopsis thaliana plant and lettuce for 5 days compared with E. coli O157:H7, and expression of pathogenesis-realted gene (PR1; induction of plant defense response) was examined by reverse transcription quantitative PCR, and potential influence of capsular polysaccharide (CPS) on the bacterial fitness on plant was investigated. Populations of E. coli O104:H4 strains (RG1, C3493, and LpfA) on Arabidopsis and lettuce were significantly (P < 0.05) greater than those of E. coli O157:H7 strains (7386 and sakai) at day 5 post-inoculation, indicating E. coli O104:H4 may have better survival ability on the plants. In addition, the E. coli O104:H4 strains produced significantly (P < 0.05) higher amounts of CPS compared with the E. coli O157:H7 strains. RG1 strain (1.5-fold) initiated significantly (P < 0.05) lower expression of PR1 gene indicating induction of plant defense response compared with E. coli O157:H7 strains 7386 (2.9-fold) and sakai (2.7-fold). Collectively, the results in this study suggests that different level of CPS production and plant defense response initiated by each STEC strain might influence the bacterial survival or persistence on plants. The present study provides better understanding of survival behavior of STEC, particularly E. coli O104:H4, using a model plant and vegetable under pre-harvest conditions with plant defense response.

Influence of surface polysaccharides of Escherichia coli O157:H7 on plant defense response and survival of the human enteric pathogen on Arabidopsis thaliana and lettuce (Lactuca sativa)

This study aimed to determine the influence of bacterial surface polysaccharides (cellulose, colanic acid, and lipopolysaccharide; LPS) on the colonization or survival of Escherichia coli O157:H7 on plants and the plant defense response. Survival of E. coli O157:H7 were evaluated on Arabidopsis thaliana and romaine lettuce as a model plant and an edible crop (leafy vegetable), respectively. The population of the wild-type strain of E. coli O157:H7 on Arabidopsis plants and lettuce was significantly (P < 0.05) greater compared with the colanic acid-deficient and LPS-truncated mutants on day 1 and day 5 post-inoculation. This result indicates that colanic acid and LPS structures may contribute to the ability of bacterial survival or persistence on plants. The wild-type strain of E. coli O157:H7 produced approximately twice the amount (P < 0.05) of capsular polysaccharide (CPS) than the colanic acid and LPS-truncated mutants. The significantly lower production of CPS was associated with significantly greater (2-fold) expression of pathogenesis-related gene (PR1) compared with the wild-type and cellulose-deficient mutant (P < 0.05). Collectively, the results of this study may suggest that specific surface polysaccharides of E. coli O157:H7 differentially induce the plant defense response, consequently affecting the survival of the human pathogen on plants. The survival and persistence of E. coli O157:H7 was similar on Arabidopsis and lettuce regardless of day post-inoculation.

Assessing the Ability of Salmonella enterica to Translocate Type III Effectors Into Plant Cells

Salmonella enterica serovar Typhimurium, a human enteric pathogen, has the ability to multiply and survive endophytically in plants. Genes encoding the type III secretion system (T3SS) or its effectors (T3Es) may contribute to its colonization. Two reporter plasmids for T3E translocation into plant cells that are based on hypersensitive response domains of avirulence proteins from the Pantoea agglomerans-beet and Xanthomonas euvesicatoria-pepper pathosystems were employed in this study to investigate the role of T3Es in the interaction of Salmonella ser. Typhimurium 14028 with plants. The T3Es of Salmonella ser. Typhimurium, SipB and SifA, which are translocated into animal cells, could not be delivered by Salmonella ser. Typhimurium into cells of beet roots or pepper leaves. In contrast, these effectors were translocated into plant cells by the phytopathogenic bacteria P. agglomerans pv. betae, Erwinia amylovora, and X. euvesicatoria. Similarly, HsvG, a T3E of P. agglomerans pv. gypsophilae, and XopAU of X. euvesicatoria could be translocated into beet roots and pepper leaves, respectively, by the plant pathogens but not by Salmonella ser. Typhimurium. Mutations in Salmonella ser. Typhimurium T3SS genes invA, ssaV, sipB, or sifA, did not affect its endophytic colonization of lettuce leaves, supporting the notion that S. enterica cannot translocate T3Es into plant cells.

Dual Expression of the Salmonella Effector SrfJ in Mammalian Cells and Plants

SrfJ is an effector of the Salmonella pathogenicity island 2-encoded type III secretion system. Salmonella enterica serovar Typhimurium expresses srfJ under two disparate sets of conditions: media with low Mg2+ and low pH, imitating intravacuolar conditions, and media with myo-inositol (MI), a carbohydrate that can be used by Salmonella as sole carbon source. We investigated the molecular basis for this dual regulation. Here, we provide evidence for the existence of two distinct promoters that control the expression of srfJ. A proximal promoter, PsrfJ, responds to intravacuolar signals and is positively regulated by SsrB and PhoP and negatively regulated by RcsB. A second distant promoter, PiolE, is negatively regulated by the MI island repressor IolR. We also explored the in vivo activity of these promoters in different hosts. Interestingly, our results indicate that the proximal promoter is specifically active inside mammalian cells whereas the distant one is expressed upon Salmonella colonization of plants. Importantly, we also found that inappropriate expression of srfJ leads to reduced proliferation inside macrophages whereas lack of srfJ expression increases survival and decreases activation of defense responses in plants. These observations suggest that SrfJ is a relevant factor in the interplay between Salmonella and hosts of different kingdoms.

Plant-mediated restriction of Salmonella enterica on tomato and spinach leaves colonized with Pseudomonas plant growth-promoting rhizobacteria

Reducing Salmonella enterica association with plants during crop production could reduce risks of fresh produce-borne salmonellosis. Plant growth-promoting rhizobacteria (PGPR) colonizing plant roots are capable of promoting plant growth and boosting resistance to disease, but the effects of PGPR on human pathogen-plant associations are not known. Two root-colonizing Pseudomonas strains S2 and S4 were investigated in spinach, lettuce and tomato for their plant growth-promoting properties and their influence on leaf populations of S. enterica serovar Newport. Plant roots were inoculated with Pseudomonas in the seedling stage. At four (tomato) and six (spinach and lettuce) weeks post-germination, plant growth promotion was assessed by shoot dry weight (SDW) and leaf chlorophyll content measurements. Leaf populations of S. Newport were measured after 24h of leaf inoculation with this pathogen by direct plate counts on Tryptic Soy Agar. Root inoculation of spinach cv. 'Tyee', with Pseudomonas strain S2 or S4 resulted in a 69% and 63% increase in SDW compared to non-inoculated controls (p<0.005 and p<0.01, respectively). Similarly, Romaine lettuce cv. 'Parris Island Cos' responded positively to S2 and S4 inoculation (53% and 48% SDW increase, respectively; p<0.05), and an increase in leaf chlorophyll content (p<0.001), compared to controls. Tomato cv. 'Nyagous' yielded significantly greater SDW (74%, p<0.01 and 54%, p<0.05 for S2 and S4, respectively), and also higher leaf chlorophyll content (19% and 29%, p<0.001, respectively) relative to controls. Leaf chlorophyll content only increased in S4-inoculated tomato cv. 'Moneymaker' plants (27%, p<0.001), although both S2 and S4 promoted plant growth by over 40% compared to controls (p<0.01 and p<0.05, respectively). No significant growth promotion was detected in tomato cv. 'BHN602', but S2-inoculated plants had elevated leaf chlorophyll content (13%, p<0.01). Root inoculation with Pseudomonas S4 restricted S. Newport populations inoculated on leaves of spinach (p<0.001) and all three tomato cultivars (p<0.05), compared to controls, 24h post Salmonella inoculation. Impairment of S. Newport leaf populations was also observed on spinach when plant roots were inoculated with S2 (p<0.01). With an initial leaf inoculum of approximately 6.0logCFU of S. Newport/plant, the significantly greater reduction of S. Newport populations on Pseudomonas-treated plants than those on non-inoculated control plants after 24h was modest with differences of one log or less. By contrast, the survival of S. Newport on the leaves of Romaine lettuce was not influenced by Pseudomonas root colonization. These findings provide evidence that root inoculation of certain specialty crops with beneficial Pseudomonas strains exhibiting PGPR properties may not only promote plant growth, but also reduce the fitness of epiphytic S. enterica in the phyllosphere. Plant-mediated effects induced by PGPR may be an effective strategy to minimize contamination of crops with S. enterica during cultivation.

Prevalence of plant beneficial and human pathogenic bacteria isolated from salad vegetables in India

BACKGROUND

The study aimed at enumerating, identifying and categorizing the endophytic cultivable bacterial community in selected salad vegetables (carrot, cucumber, tomato and onion). Vegetable samples were collected from markets of two vegetable hot spot growing areas, during two different crop harvest seasons. Crude and diluted vegetable extracts were plated and the population of endophytic bacteria was assessed based on morphologically distinguishable colonies. The bacterial isolates were identified by growth in selective media, biochemical tests and 16S rRNA gene sequencing.

RESULTS

The endophytic population was found to be comparably higher in cucumber and tomato in both of the sampling locations, whereas lower in carrot and onion. Bacterial isolates belonged to 5 classes covering 46 distinct species belonging to 19 genera. Human opportunistic pathogens were predominant in carrot and onion, whereas plant beneficial bacteria dominated in cucumber and tomato. Out of the 104 isolates, 16.25% are human pathogens and 26.5% are human opportunistic pathogens.

CONCLUSIONS

Existence of a high population of plant beneficial bacteria was found to have suppressed the population of plant and human pathogens. There is a greater potential to study the native endophytic plant beneficial bacteria for developing them as biocontrol agents against human pathogens that are harboured by plants.

Interactions of Salmonella with animals and plants

Salmonella enterica species are Gram-negative bacteria, which are responsible for a wide range of food- and water-borne diseases in both humans and animals, thereby posing a major threat to public health. Recently, there has been an increasing number of reports, linking Salmonella contaminated raw vegetables and fruits with food poisoning. Many studies have shown that an essential feature of the pathogenicity of Salmonella is its capacity to cross a number of barriers requiring invasion of a large variety of cells and that the extent of internalization may be influenced by numerous factors. However, it is poorly understood how Salmonella successfully infects hosts as diversified as animals or plants. The aim of this review is to describe the different stages required for Salmonella interaction with its hosts: (i) attachment to host surfaces; (ii) entry processes; (iii) multiplication; (iv) suppression of host defense mechanisms; and to point out similarities and differences between animal and plant infections.

Bacterial flagella: twist and stick, or dodge across the kingdoms

The flagellum organelle is an intricate multiprotein assembly best known for its rotational propulsion of bacteria. However, recent studies have expanded our knowledge of other functions in pathogenic contexts, particularly adherence and immune modulation, e.g., for Salmonella enterica, Campylobacter jejuni, Pseudomonas aeruginosa, and Escherichia coli. Flagella-mediated adherence is important in host colonisation for several plant and animal pathogens, but the specific interactions that promote flagella binding to such diverse host tissues has remained elusive. Recent work has shown that the organelles act like probes that find favourable surface topologies to initiate binding. An emerging theme is that more general properties, such as ionic charge of repetitive binding epitopes and rotational force, allow interactions with plasma membrane components. At the same time, flagellin monomers are important inducers of plant and animal innate immunity: variation in their recognition impacts the course and outcome of infections in hosts from both kingdoms. Bacteria have evolved different strategies to evade or even promote this specific recognition, with some important differences shown for phytopathogens. These studies have provided a wider appreciation of the functions of bacterial flagella in the context of both plant and animal reservoirs.

N-acyl-homoserine lactones-producing bacteria protect plants against plant and human pathogens

The implementation of beneficial microorganisms for plant protection has a long history. Many rhizobia bacteria are able to influence the immune system of host plants by inducing resistance towards pathogenic microorganisms. In this report, we present a translational approach in which we demonstrate the resistance-inducing effect of Ensifer meliloti (Sinorhizobium meliloti) on crop plants that have a significant impact on the worldwide economy and on human nutrition. Ensifer meliloti is usually associated with root nodulation in legumes and nitrogen fixation. Here, we suggest that the ability of S. meliloti to induce resistance depends on the production of the quorum-sensing molecule, oxo-C14-HSL. The capacity to enhanced resistance provides a possibility to the use these beneficial bacteria in agriculture. Using the Arabidopsis-Salmonella model, we also demonstrate that the application of N-acyl-homoserine lactones-producing bacteria could be a successful strategy to prevent plant-originated infections with human pathogens.

Image-based Analysis to Study Plant Infection with Human Pathogens

Our growing awareness that contaminated plants, fresh fruits and vegetables are responsible for a significant proportion of food poisoning with pathogenic microorganisms indorses the demand to understand the interactions between plants and human pathogens. Today we understand that those pathogens do not merely survive on or within plants, they actively infect plant organisms by suppressing their immune system. Studies on the infection process and disease development used mainly physiological, genetic, and molecular approaches, and image-based analysis provides yet another method for this toolbox. Employed as an observational tool, it bears the potential for objective and high throughput approaches, and together with other methods it will be very likely a part of data fusion approaches in the near future.

The Salmonella effector protein SpvC, a phosphothreonine lyase is functional in plant cells

Salmonella is one of the most prominent causes of food poisoning and growing evidence indicates that contaminated fruits and vegetables are an increasing concern for human health. Successful infection demands the suppression of the host immune system, which is often achieved via injection of bacterial effector proteins into host cells. In this report we present the function of Salmonella effector protein in plant cell, supporting the new concept of trans-kingdom competence of this bacterium. We screened a range of Salmonella Typhimurium effector proteins for interference with plant immunity. Among these, the phosphothreonine lyase SpvC attenuated the induction of immunity-related genes when present in plant cells. Using in vitro and in vivo systems we show that this effector protein interacts with and dephosphorylates activated Arabidopsis Mitogen-activated Protein Kinase 6 (MPK6), thereby inhibiting defense signaling. Moreover, the requirement of Salmonella SpvC was shown by the decreased proliferation of the ΔspvC mutant in Arabidopsis plants. These results suggest that some Salmonella effector proteins could have a conserved function during proliferation in different hosts. The fact that Salmonella and other Enterobacteriaceae use plants as hosts strongly suggests that plants represent a much larger reservoir for animal pathogens than so far estimated.

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.

Ethylene signalling affects susceptibility of tomatoes to Salmonella

Fresh fruits and vegetables are increasingly recognized as important reservoirs of human pathogens, and therefore, significant attention has been directed recently to understanding mechanisms of the interactions between plants and enterics, like Salmonella. A screen of tomato cultivars for their susceptibility to Salmonella revealed significant differences in the ability of this human pathogen to multiply within fruits; expression of the Salmonella genes (cysB, agfB, fadH) involved in the interactions with tomatoes depended on the tomato genotype and maturity stage. Proliferation of Salmonella was strongly reduced in the tomato mutants with defects in ethylene synthesis, perception and signal transduction. While mutation in the ripening-related ethylene receptor Nr resulted only in a modest reduction in Salmonella numbers within tomatoes, strong inhibition of the Salmonella proliferation was observed in rin and nor tomato mutants. RIN and NOR are regulators of ethylene synthesis and ripening. A commercial tomato variety heterozygous for rin was less susceptible to Salmonella under the greenhouse conditions but not when tested in the field over three production seasons.

Salmonella enterica induces and subverts the plant immune system

Infections with Salmonella enterica belong to the most prominent causes of food poisoning and infected fruits and vegetables represent important vectors for salmonellosis. Although it was shown that plants raise defense responses against Salmonella, these bacteria persist and proliferate in various plant tissues. Recent reports shed light into the molecular interaction between plants and Salmonella, highlighting the defense pathways induced and the means used by the bacteria to escape the plant immune system and accomplish colonization. It was recently shown that plants detect Salmonella pathogen-associated molecular patterns (PAMPs), such as the flagellin peptide flg22, and activate hallmarks of the defense program known as PAMP-triggered immunity (PTI). Interestingly, certain Salmonella strains carry mutations in the flg22 domain triggering PTI, suggesting that a strategy of Salmonella is to escape plant detection by mutating PAMP motifs. Another strategy may rely on the type III secretion system (T3SS) as T3SS mutants were found to induce stronger plant defense responses than wild type bacteria. Although Salmonella effector delivery into plant cells has not been shown, expression of Salmonella effectors in plant tissues shows that these bacteria also possess powerful means to manipulate the plant immune system. Altogether, these data suggest that Salmonella triggers PTI in plants and evolved strategies to avoid or subvert plant immunity.