Plant - Salmonella or E. coli Interactions
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Antimicrobial effect of lauroyl arginate ethyl on Escherichia coli O157:H7 and Listeria monocytogenes on red oak leaf lettuce

The objective of the present study was to determine the antimicrobial activity of lauroyl arginate ethyl (LAE) against enterohemorrhagic Escherichia coli (EHEC) O157:H7 and Listeria monocytogenes on oak leaf lettuce. Thus, bacterial suspensions of both pathogens containing 9.0 log10 CFU/mL were spot inoculated on the upper surfaces of lettuce leaves and the leaves were washed with water containing LAE in a final concentration of 100 mg/L. The viable counts of L. monocytogenes DSM 20600T and Escherichia coli O157:H7 strain EDL933 were reduced with this treatment by ~3.5 log10 CFU/g and ~2.5 log10 CFU/g, respectively. The microbial load of the wash water was reduced by more than 4.1 log10 CFU/mL and was below the detection limit. The second objective was to study whether adherence factors of EHEC O157:H7 strain EDL933 influence the ability to adhere on the lettuce surface, as well as the effectivity of the washing processes. Therefore, the flagellin gene fliC and the pilin subunit encoding gene hcpA of the hemorrhagic coli pilus were deleted. Based on the initial inoculation level, and without any washing step, the hcpA mutant was recovered 18 % less than the wild type from the leaf surface, the recovery of the fliC mutant was approximately 30 % higher as observed for the wild type. Both mutants could be washed from the lettuce leaves to a similar level as the wild type (~2.7 log10 CFU/g with LAE treatment and ~1.0 log10 CFU/g without LAE treatment). The findings of this study help to develop novel intervention strategies for fresh produce processing and washing treatments.

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Prediction of bacterial associations with plants using a supervised machine-learning approach

Recent scenarios of fresh produce contamination by human enteric pathogens have resulted in severe food-borne outbreaks, and a new paradigm has emerged stating that some human-associated bacteria can use plants as secondary hosts. As a consequence, there has been growing concern in the scientific community about these interactions that have not yet been elucidated. Since this is a relatively new area, there is a lack of strategies to address the problem of food-borne illnesses due to the ingestion of fruits and vegetables. In the present study, we performed specific genome annotations to train a supervised machine-learning model that allows for the identification of plant-associated bacteria with a precision of ∼93%. The application of our method to approximately 9,500 genomes predicted several unknown interactions between well-known human pathogens and plants, and it also confirmed several cases for which evidence has been reported. We observed that factors involved in adhesion, the deconstruction of the plant cell wall and detoxifying activities were highlighted as the most predictive features. The application of our strategy to sequenced strains that are involved in food poisoning can be used as a primary screening tool to determine the possible causes of contaminations.
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Investigating Metrics Proposed to Prevent the Harvest of Leafy Green Crops Exposed to Floodwater Contaminated with Escherichia coli

The California Leafy Green Products Handler Marketing Agreement (LGMA) requires leafy green crops within 9 m of the edge of a flooded field not be harvested due to potential contamination. Further, previously flooded soils should not be replanted for 60 days. In this study, the suitability of the LGMA metrics for farms in the Mid-Atlantic region of the United States was evaluated. The upper end of a spinach bed (Beltsville, MD) established on a -5% grade was flooded with water containing 6 log CFU/mL Escherichia coli to model a worst-case scenario of bacterial movement through soil. Escherichia coli prevalence in soil and on foliar tissue was determined by Most Probable Number (MPN) analysis at distances up to 9 m from the edge of the flood for 63 days. While E. coli was quickly detected at the 9 m distance within 1 day in the spring trial and 3 days in the fall trial, no E. coli were detected on plants outside the flood zone after 14 days. On day 63, E. coli populations in the flood zone soil were higher in the fall than in the spring. Regression analysis predicted the time required for a 3-log MPN/gdw decrease in E. coli populations inside the flood zone was within the 60-day LGMA guideline in the spring, but would require 90 days in the fall. Overall, data suggest that the current guidelines should be revised to include considerations of field and weather conditions that may promote bacterial movement and survival.
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Microbiological Food Safety Status of Commercially Produced Tomatoes from Production to Marketing

Tomatoes have been implicated in various microbial disease outbreaks and are considered a potential vehicle for foodborne pathogens. Traceback studies mostly implicate contamination during production and/or processing. The microbiological quality of commercially produced tomatoes was thus investigated from the farm to market, focusing on the impact of contaminated irrigation and washing water, facility sanitation, and personal hygiene. A total of 905 samples were collected from three largescale commercial farms from 2012 through 2014. The farms differed in water sources used (surface versus well) and production methods (open field versus tunnel). Levels of total coliforms and Escherichia coli and prevalence of E. coli O157:H7 and Salmonella Typhimurium were determined. Dominant coliforms were identified using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. No pathogens or E. coli were detected on any of the tomatoes tested throughout the study despite the high levels of coliforms (4.2 to 6.2 log CFU/g) present on the tomatoes at the market. The dominant species associated with tomatoes belonged to the genera Enterobacter, Klebsiella, and Citrobacter. Water used on the farm for irrigation considered not fit for purpose according to national agricultural irrigation standards, with high E. coli levels resulting from either a highly contaminated source water (river water at 3.19 log most probable number [MPN]/100 ml) or improper storage of source water (stored well water at 1.72 log MPN/100 ml). Salmonella Typhimurium was detected on two occasions on a contact surface in the processing facility of the first farm in 2012. Contact surface coliform counts were 2.9 to 4.8 log CFU/cm2. Risk areas identified in this study were water used for irrigation and poor sanitation practices in the processing facility. Implementation of effective food safety management systems in the fresh produce industry is of the utmost importance to ensure product safety for consumers.
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Diguanylate Cyclases AdrA and STM1987 Regulate Salmonella enterica Exopolysaccharide Production during Plant Colonization in an Environment-Dependent Manner

Increasing evidence indicates that despite exposure to harsh environmental stresses, Salmonella enterica successfully persists on plants, utilizing fresh produce as a vector to animal hosts. Among the important S. enterica plant colonization factors are those involved in biofilm formation. S. enterica biofilm formation is controlled by the signaling molecule cyclic di-GMP and represents a sessile lifestyle on surfaces that protects the bacterium from environmental factors. Thus, the transition from a motile, planktonic lifestyle to a sessile lifestyle may represent a vital step in bacterial success. This study examined the mechanisms of S. enterica plant colonization, including the role of diguanylate cyclases (DGCs) and phosphodiesterases (PDEs), the enzymes involved in cyclic di-GMP metabolism. We found that two biofilm components, cellulose and curli, are differentially required at distinct stages in root colonization and that the DGC STM1987 regulates cellulose production in this environment independent of AdrA, the DGC that controls the majority of in vitro cellulose production. In addition, we identified a new function for AdrA in the transcriptional regulation of colanic acid and demonstrated that adrA and colanic acid biosynthesis are associated with S. enterica desiccation tolerance on the leaf surface. Finally, two PDEs with known roles in motility, STM1344 and STM1697, had competitive defects in the phyllosphere, suggesting that regulation of motility is crucial for S. enterica survival in this niche. Our results indicate that specific conditions influence the contribution of individual DGCs and PDEs to bacterial success, perhaps reflective of differential responses to environmental stimuli.
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Great paper from Jeri Barak's lab.

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News release: E. coli O157 cases linked to alfalfa sprouts

News release: E. coli O157 cases linked to alfalfa sprouts | Plant - Salmonella or E. coli Interactions | Scoop.it
State health and agriculture officials are investigating an outbreak of foodborne illness associated with alfalfa sprouts produced by Jack & The Green Sprouts. Retailers and restaurants should not sell or serve alfalfa sprouts produced by Jack & The Green Sprouts, and consumers should not eat them at this time.

Routine disease monitoring by the Minnesota Department of Health (MDH) identified seven E. coli O157:NM cases in January and early February; E. coli bacteria from those cases all had the same DNA fingerprint. The ill individuals range in age from 18 to 84 years, and five are female. Four of the cases are residents of the Twin Cities metro area, and three live in greater Minnesota. Two were hospitalized, and both have recovered.
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Alternative disinfection methods to chlorine for use in the fresh-cut industry

The use of chlorine as a disinfectant in the fresh-cut produce industry has been identified as a concern mainly due to public health issues. In fact, this chemical, commonly used as hypochlorous acid and hypochlorite, has already been prohibited in some European countries, due to the potential production of toxic by-products, such as chloroform and other trihalomethanes, chloramines and haloacetic acids. The search for alternative methods of disinfection is therefore a current and on-going challenge in both Academia and Industry. Some methods are well described in the literature on the disinfection of food-contact surfaces and process water and also on the decontamination of the produce. These methods are commonly classified as biological (bacteriocins, bacteriophages, enzymes and phytochemicals), chemical (chlorine dioxide, electrolyzed oxidizing water, hydrogen peroxide, ozone, organic acids, etc) and physical (irradiation, filtration, ultrasounds, ultraviolet light, etc). This review provides updated information on the state of the art of the available disinfection strategies alternative to chlorine that can be used in the fresh-cut industry. The use of combined methods to replace and/or reduce the use of chlorine is also reviewed.
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Isolation, Characterization and Identification of Lactic acid Bacteria from Lettuce and their Efficacy against Pathogenic Bacteria

Biopreservation has recently been endorsed as an alternative to reduce microbial risks and conserve fresh products. This study focused on the importance of lactic acid bacteria (LAB) and its efficiency as a bioprotective agent. One hundred and fifty LAB strains were isolated from iceberg lettuce, characterized and identified using 16S rDNA sequencing and API 50CH. All strains displayed a considerable antibacterial activity against pathogenic bacteria namely Escherichia coli, Listeria monocytogenes, Pseudomonas aeruginosa, Salmonella typhimurium and Staphylococcus aureus. All the isolates were found to correspond to Lactobacillus plantarum strain. The results deduced that the production of organic acids was the most frequent mechanism of inhibition used among the tested isolates. Furthermore, it was proven that LAB strain harmed the growth of two foodborne pathogen bacteria, S. typhimurium and L. monocytogenes, in shredded iceberg lettuce.
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Abiotic Stress and Phyllosphere Bacteria Influence the Survival of Human Norovirus and Its Surrogates on Preharvest Leafy Greens

Foodborne outbreaks of human noroviruses (HuNoVs) are frequently associated with leafy greens. Because there is no effective method to eliminate HuNoV from postharvest leafy greens, understanding virus survival under preharvest conditions is crucial. The objective of this study was to evaluate the survival of HuNoV and its surrogate viruses, murine norovirus (MNV), porcine sapovirus (SaV), and Tulane virus (TV), on preharvest lettuce and spinach that were subjected to abiotic stress (physical damage, heat, or flood). We also examined the bacteria culturable from the phyllosphere in response to abiotic stress and in relation to viral persistence. Mature plants were subjected to stressors 2 days prior to inoculation of the viruses on leaves. We quantified the viral RNA, determined the infectivity of the surrogates, and performed bacterial counts on postinoculation days (PIDs) 0, 1, 7, and 14. For both plant types, time exerted significant effects on HuNoV, MNV, SaV, and TV RNA titers, with greater effects being seen for the surrogates. Infectious surrogate viruses were undetectable on PID 14. Only physical damage on PID 14 significantly enhanced HuNoV RNA persistence on lettuce, while the three stressors differentially enhanced the persistence of MNV and TV RNA. Bacterial counts were significantly affected by time and plant type but not by the stressors. However, bacterial counts correlated significantly with HuNoV RNA titers on spinach and with the presence of surrogate viruses on both plant types under various conditions. In conclusion, abiotic stressors and phyllosphere bacterial density may differentially influence the survival of HuNoV and its surrogates on lettuce and spinach, emphasizing the need for the use of preventive measures at the preharvest stage.
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Bacterial community and proteome analysis of fresh-cut lettuce as affected by packaging

With the growing demand of fresh-cut vegetables, a variety of packaging films are produced specifically to improve safety and quality of the fresh vegetables over the storage period. The aim of our work was to evaluate the influence of different packaging films on the quality of fresh-cut lettuce analyzing changes in bacterial community composition and modifications at the proteome level, by means of culture-dependent/culture-independent methods and differential gel electrophoresis combined with mass spectrometry analysis. Total viable counts indicated the presence of a highly variable and complex microbial flora, around a mean value of 6.26 log10 CFU g−1. Analysis of terminal-restriction fragment length polymorphism data indicated that bacterial communities changed with packaging films and time, showing differences in community composition and diversity indices between the commercially available package (F) and the new packages (A and C), in the first days after packaging. Also proteomic analysis revealed significant changes, involving proteins related to energy metabolism, photosynthesis, plant defense and oxidative stress processes, between F and A/C packages. In conclusion, microbiological and proteomic analysis have proved to be powerful tools to provide new insights into both the composition of leaf-associated bacterial communities and protein content of fresh-cut lettuce during the shelf-life storage process.
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All is number? A faster way to analyse plant root images

All is number? A faster way to analyse plant root images | Plant - Salmonella or E. coli Interactions | Scoop.it
It’s said that Pythagoras once claimed that everything is fun­da­ment­ally num­ber. He found numer­ical rela­tion­ships in things that seems a world away from math­em­at­ics. Two and a half thou­sand years later we’re happy with quan­ti­fy­ing things, but it can still be hard. For example, Berntson back in 1994 looked at the prob­lems of describ­ing a root’s fractal dimen­sions. Roots can be messy so how can you ana­lyse them rap­idly — or bet­ter still how can you auto­mate it? RootGraph: a graphic optim­iz­a­tion tool for auto­mated image ana­lysis of plant roots by Cai et al puts for­ward a new technique.
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Global Transcriptome and Mutagenic Analyses of the Acid Tolerance Response of Salmonella enterica Serovar Typhimurium

Global Transcriptome and Mutagenic Analyses of the Acid Tolerance Response of Salmonella enterica Serovar Typhimurium | Plant - Salmonella or E. coli Interactions | Scoop.it
Salmonella enterica serovar Typhimurium (S. Typhimurium) is one of the leading causative agents of food-borne bacterial gastroenteritis. Swift invasion through the intestinal tract and successful establishment in systemic organs are associated with the adaptability of S. Typhimurium to different stress environments. Low-pH stress serves as one of the first lines of defense in mammalian hosts, which S. Typhimurium must efficiently overcome to establish an infection. Therefore, a better understanding of the molecular mechanisms underlying the adaptability of S. Typhimurium to acid stress is highly relevant. In this study, we have performed a transcriptome analysis of S. Typhimurium under the acid tolerance response (ATR) and found a large number of genes (∼47%) to be differentially expressed (more than 1.5-fold or less than −1.5-fold; P < 0.01). Functional annotation revealed differentially expressed genes to be associated with regulation, metabolism, transport and binding, pathogenesis, and motility. Additionally, our knockout analysis of a subset of differentially regulated genes facilitated the identification of proteins that contribute to S. Typhimurium ATR and virulence. Mutants lacking genes encoding the K+ binding and transport protein KdpA, hypothetical protein YciG, the flagellar hook cap protein FlgD, and the nitrate reductase subunit NarZ were significantly deficient in their ATRs and displayed varied in vitro virulence characteristics. This study offers greater insight into the transcriptome changes of S. Typhimurium under the ATR and provides a framework for further research on the subject.
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Plant Pathogen-Induced Water-Soaking Promotes Salmonella enterica Growth on Tomato Leaves

Plant Pathogen-Induced Water-Soaking Promotes Salmonella enterica Growth on Tomato Leaves | Plant - Salmonella or E. coli Interactions | Scoop.it
Plant pathogen infection is a critical factor for the persistence of Salmonella enterica on plants. We investigated the mechanisms responsible for the persistence of S. enterica on diseased tomato plants by using four diverse bacterial spot Xanthomonas species that differ in disease severities. Xanthomonas euvesicatoria and X. gardneri infection fostered S. enterica growth, while X. perforans infection did not induce growth but supported the persistence of S. enterica. X. vesicatoria-infected leaves harbored S. enterica populations similar to those on healthy leaves. Growth of S. enterica was associated with extensive water-soaking and necrosis in X. euvesicatoria- and X. gardneri-infected plants. The contribution of water-soaking to the growth of S. enterica was corroborated by an increased growth of populations on water-saturated leaves in the absence of a plant pathogen. S. enterica aggregates were observed with bacterial spot lesions caused by either X. euvesicatoria or X. vesicatoria; however, more S. enterica aggregates formed on X. euvesicatoria-infected leaves as a result of larger lesion sizes per leaf area and extensive water-soaking. Sparsely distributed lesions caused by X. vesicatoria infection do not support the overall growth of S. enterica or aggregates in areas without lesions or water-soaking; S. enterica was observed as single cells and not aggregates. Thus, pathogen-induced water-soaking and necrosis allow S. enterica to replicate and proliferate on tomato leaves. The finding that the pathogen-induced virulence phenotype affects the fate of S. enterica populations in diseased plants suggests that targeting of plant pathogen disease is important in controlling S. enterica populations on plants.
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Whole-Transcriptome Analysis of Verocytotoxigenic Escherichia coli O157:H7 (Sakai) Suggests Plant-Species-Specific Metabolic Responses on Exposure to Spinach and Lettuce Extracts

Whole-Transcriptome Analysis of Verocytotoxigenic Escherichia coli O157:H7 (Sakai) Suggests Plant-Species-Specific Metabolic Responses on Exposure to Spinach and Lettuce Extracts | Plant - Salmonella or E. coli Interactions | Scoop.it
Verocytotoxigenic Escherichia coli (VTEC) can contaminate crop plants, potentially using them as secondary hosts, which can lead to food-borne infection. Currently, little is known about the influence of the specific plant species on the success of bacterial colonization. As such, we compared the ability of the VTEC strain, E. coli O157:H7 ‘Sakai,’ to colonize the roots and leaves of four leafy vegetables: spinach (Spinacia oleracea), lettuce (Lactuca sativa), vining green pea (Pisum sativum), and prickly lettuce (Lactuca serriola), a wild relative of domesticated lettuce. Also, to determine the drivers of the initial response on interaction with plant tissue, the whole transcriptome of E. coli O157:H7 Sakai was analyzed following exposure to plant extracts of varying complexity (spinach leaf lysates or root exudates, and leaf cell wall polysaccharides from spinach or lettuce). Plant extracts were used to reduce heterogeneity inherent in plant–microbe interactions and remove the effect of plant immunity. This dual approach provided information on the initial adaptive response of E. coli O157:H7 Sakai to the plant environment together with the influence of the living plant during bacterial establishment and colonization. Results showed that both the plant tissue type and the plant species strongly influence the short-term (1 h) transcriptional response to extracts as well as longer-term (10 days) plant colonization or persistence. We show that propagation temperature (37 vs. 18°C) has a major impact on the expression profile and therefore pre-adaptation of bacteria to a plant-relevant temperature is necessary to avoid misleading temperature-dependent wholescale gene-expression changes in response to plant material. For each of the plant extracts tested, the largest group of (annotated) differentially regulated genes were associated with metabolism. However, large-scale differences in the metabolic and biosynthetic pathways between treatment types indicate specificity in substrate utilization. Induction of stress-response genes reflected the apparent physiological status of the bacterial genes in each extract, as a result of glutamate-dependent acid resistance, nutrient stress, or translational stalling. A large proportion of differentially regulated genes are uncharacterized (annotated as hypothetical), which could indicate yet to be described functional roles associated with plant interaction for E. coli O157:H7 Sakai.

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Chitin Mixed in Potting Soil Alters Lettuce Growth, the Survival of Zoonotic Bacteria on the Leaves and Associated Rhizosphere

Chitin Mixed in Potting Soil Alters Lettuce Growth, the Survival of Zoonotic Bacteria on the Leaves and Associated Rhizosphere | Plant - Salmonella or E. coli Interactions | Scoop.it
Chitin is a promising soil amendment for improving soil quality, plant growth, and plant resilience. The objectives of this study were twofold. First, to study the effect of chitin mixed in potting soil on lettuce growth and on the survival of two zoonotic bacterial pathogens, Escherichia coli O157:H7 and Salmonella enterica on the lettuce leaves. Second, to assess the related changes in the microbial lettuce rhizosphere, using phospholipid fatty acid (PLFA) analysis and amplicon sequencing of a bacterial 16S rRNA gene fragment and the fungal ITS2. As a result of chitin addition, lettuce fresh yield weight was significantly increased. S. enterica survival in the lettuce phyllosphere was significantly reduced. The E. coli O157:H7 survival was also lowered, but not significantly. Moreover, significant changes were observed in the bacterial and fungal community of the lettuce rhizosphere. PLFA analysis showed a significant increase in fungal and bacterial biomass. Amplicon sequencing showed no increase in fungal and bacterial biodiversity, but relative abundances of the bacterial phyla Acidobacteria, Verrucomicrobia, Actinobacteria, Bacteroidetes, and Proteobacteria and the fungal phyla Ascomycota, Basidiomycota, and Zygomycota were significantly changed. More specifically, a more than 10-fold increase was observed for operational taxonomic units belonging to the bacterial genera Cellvibrio, Pedobacter, Dyadobacter, and Streptomyces and to the fungal genera Lecanicillium and Mortierella. These genera include several species previously reported to be involved in biocontrol, plant growth promotion, the nitrogen cycle and chitin degradation. These results enhance the understanding of the response of the rhizosphere microbiome to chitin amendment. Moreover, this is the first study to investigate the use of soil amendments to control the survival of S. enterica on plant leaves.
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Environmental metabolomics of the plant surface provides insights on Salmonella enterica colonization of tomato

Foodborne illness-causing enteric bacteria are able to colonize plant surfaces without causing infection. We lack understanding of how epiphytic persistence of enteric bacteria occurs on plants, possibly as an adaptive transit strategy to maximize chances of re-entering herbivorous hosts. We used tomato (Solanum lycopersicum) cultivars that have exhibited differential susceptibilities to Salmonella enterica colonization to investigate the influence of plant surface compounds and exudates on enteric bacterial populations. Tomato fruit, shoot and root exudates collected at different developmental stages supported growth of S. enterica to various degrees in a cultivar and plant organ dependent manner. S. enterica growth in fruit exudates of various cultivars correlated with epiphytic growth data (R2=0.504; p=0.006), providing evidence that plant surface compounds drive bacterial colonization success. Chemical profiling of tomato surface compounds with gas chromatography time-of-flight mass spectrometry (GC-TOF-MS) provided valuable information about the metabolic environment on fruit, shoot and root surfaces. Hierarchical cluster analysis of the data revealed quantitative differences in phytocompounds among cultivars, and changes over a developmental course and by plant organ (p<0.002). Sugars, sugar alcohols and organic acids were associated with increased S. enterica growth, while fatty acids, including palmitic and oleic acids, were negatively correlated. We demonstrate that the plant surface metabolite landscape has a significant impact on S. enterica growth and colonization efficiency. This environmental metabolomics approach provides an avenue to understand interactions between human pathogens and plants that could lead to strategies to identify or breed crop cultivars for microbiologically safer produce.
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Effects of Cover Crop Species and Season on Population Dynamics of Escherichia coli and Listeria innocua in Soil

Cover crops provide several ecosystem services, but their impact on enteric bacterial survival remains unexplored. The influence of cover cropping on foodborne pathogen indicator bacteria was assessed in five cover crop/green manure systems: cereal rye, hairy vetch, crimson clover, hairy vetch-rye and crimson clover-rye mixtures, and bare ground. Cover crop plots were inoculated with Escherichia coli and Listeria innocua in the fall of 2013 and 2014 and tilled into the soil in the spring to form green manure. Soil samples were collected and the bacteria enumerated. Time was a factor for all bacterial populations studied in all fields (P < 0.001). E. coli levels declined when soil temperatures dipped to <5°C and were detected only sporadically the following spring. L. innocua diminished somewhat but persisted, independently of season. In an organic field, the cover crop was a factor for E. coli in year 1 (P = 0.004) and for L. innocua in year 2 (P = 0.011). In year 1, E. coli levels were highest in the rye and hairy vetch-rye plots. In year 2, L. innocua levels were higher in hairy vetch-rye (P = 0.01) and hairy vetch (P = 0.03) plots than in the rye plot. Bacterial populations grew (P < 0.05) or remained the same 4 weeks after green manure incorporation, although initial reductions in L. innocua numbers were observed after tilling (P < 0.05). Green manure type was a factor only for L. innocua abundance in a transitional field (P < 0.05). Overall, the impacts of cover crops/green manures on bacterial population dynamics in soil varied, being influenced by bacterial species, time from inoculation, soil temperature, rainfall, and tillage; this reveals the need for long-term studies.
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Peer mentoring & why you should, too

Peer mentoring & why you should, too | Plant - Salmonella or E. coli Interactions | Scoop.it
Mentoring programs are believed to be essential to a successful career in science and are considered a critical step in improving the retention of women and under-represented minorities in science, engineering and technology fields*. Traditional mentoring matches a junior or inexperienced person—the mentee—with someone senior or more experienced—the mentor. The topic of today’s post is a different kind of mentoring, which I am calling “peer mentoring**”. In this case, each participant is both a mentor and a mentee. Over the last 15 years, I have been involved in several different peer-mentoring groups, and in every case they have been a powerful source of personal and professional growth. Here, I explain what I mean by peer mentoring, describe my own experiences, and list some suggestions for starting your own group.
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Synergetic antibacterial efficacy of cold nitrogen plasma and clove oil against Escherichia coli O157:H7 biofilms on lettuce

Synergetic antibacterial efficacy of cold nitrogen plasma and clove oil against Escherichia coli O157:H7 biofilms on lettuce | Plant - Salmonella or E. coli Interactions | Scoop.it
This study aims to evaluate the antimicrobial effects of cold nitrogen plasma (CNP) and clove oil against Escherichia coli O157:H7 (E.coli O157:H7) biofilm on lettuce. Both clove oil (1 mg/mL, 2 mg/mL and 4 mg/mL) and CNP (400 to 600 W) displayed significant eradication effect on E.coli O157:H7 biofilms in vitro (p < 0.001). Subsequently, the antibiofilm effect of combined treatment was studied as well. Compared with the respective treatment, combined treatment exhibited remarkable synergistic effect on eradicating E.coli O157:H7 biofilms. The confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) had also visually testified that the antibacterial effects of clove oil on E.coli O157:H7 biofilms (in vitro and on lettuce) were enhanced by CNP at 400 W for short treatment duration. The results of sensory evaluation indicated that combined treatment has mild negative effect on lettuce quality. Moreover, the synergetic antibacterial mechanism of clove oil and CNP against E. coli O157:H7 was concluded as that they could damage the bacterial cell wall and the outer membrane, leading to leakage of cellular components, such as nucleic acid and ATP.
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ANALYSIS OF THE BACTERIAL EPIPHYTIC MICROBIOTA OF OAK LEAF LETTUCE WITH 16S RIBOSOMAL RNA GENE ANALYSIS

The leaf microbiota has major influences on the quality of ready-to-eat lettuce. While studies investigating the epi- and endophytic microbiota of lettuce have been published, no protocols focusing only on the epiphytic microbiota exist. As the epiphytic microbiota may be especially influenced by technological steps in the production of ready-to-eat lettuce, an in-depth knowledge of these microorganisms is essential with regard to consumer safety and spoilage. Currently it is not clear to what extent results gained from single samples are representative of the community composition. A technique for the separation of bacterial cells from the leaf surface was applied to green oak leaf lettuce. The bacterial diversity was analysed in triplicate with high throughput Roche 454 sequencing of prokaryotic 16S rRNA genes to analyse the intra-sample variation. Sequence analysis revealed members of the phyla Acidobacteria, Actinobacteria, Bacteroidetes, Firmicutes, Gemmatimonadetes, Proteobacteria and Verrucomicrobia, and of the candidate division WYO. The ten most abundant proteobacterial genera in all three samples were Alkanindiges (24.6%), Pseudomonas (11.3%), Sphingomonas (8.6%), Janthinobacterium (8.3%), Acinetobacter (4.3%), Polaromonas (1.3%), Erwinia (1.1%), and Methylobacterium (1.1%). The genera Pedobacter (2.5%) and Hymenobacter (1.4%) dominated the phylum Bacteroidetes. The intra-sample variation was less than 0.7% for seven of these most abundant genera with the exception of Pseudomonas, Janthinobacterium and Alkanindiges, where larger standard deviations were obtained. This low intra-sample variation demonstrates that the established technique based on oak leaf lettuce is suitable for the culture-independent analysis of the epiphytic bacterial microbiota of produce.

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Influence of Vacuum Cooling on Escherichia coli O157:H7 Infiltration in Fresh Leafy Greens via a Multiphoton-Imaging Approach

Microbial pathogen infiltration in fresh leafy greens is a significant food safety risk factor. In various postharvest operations, vacuum cooling is a critical process for maintaining the quality of fresh produce. The overall goal of this study was to evaluate the risk of vacuum cooling-induced infiltration of Escherichia coli O157:H7 into lettuce using multiphoton microscopy. Multiphoton imaging was chosen as the method to locate E. coli O157:H7 within an intact lettuce leaf due to its high spatial resolution, low background fluorescence, and near-infrared (NIR) excitation source compared to those of conventional confocal microscopy. The variables vacuum cooling, surface moisture, and leaf side were evaluated in a three-way factorial study with E. coli O157:H7 on lettuce. A total of 188 image stacks were collected. The images were analyzed for E. coli O157:H7 association with stomata and E. coli O157:H7 infiltration. The quantitative imaging data were statistically analyzed using analysis of variance (ANOVA). The results indicate that the low-moisture condition led to an increased risk of microbial association with stomata (P < 0.05). Additionally, the interaction between vacuum cooling levels and moisture levels led to an increased risk of infiltration (P < 0.05). This study also demonstrates the potential of multiphoton imaging for improving sensitivity and resolution of imaging-based measurements of microbial interactions with intact leaf structures, including infiltration.
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Diguanylate cyclases, AdrA and STM1987, regulate Salmonella enterica exopolysaccharide production during plant colonization in an environment-dependent manner

Increasing evidence indicates that despite exposure to harsh environmental stresses, Salmonella enterica successfully persists on plants, utilizing fresh produce as a vector to animal hosts. Among the important S. enterica plant colonization factors are those involved in biofilm formation. S. enterica biofilm formation is controlled by the signaling molecule cyclic di-GMP and represents a sessile lifestyle on surfaces that protects the bacterium from environmental factors. Thus, the transition from a motile, planktonic lifestyle to a sessile lifestyle may represent a vital step in bacterial success. This study examined the mechanisms of S. enterica plant colonization, including the role of diguanylate cyclases (DGCs) and phosphodiesterases (PDEs), the enzymes involved in cyclic di-GMP metabolism. We found that two biofilm components, cellulose and curli, are differentially required at distinct stages in root colonization and that the DGC STM1987 regulates cellulose production in this environment independent of AdrA, the DGC that controls the majority of in vitro cellulose production. In addition, we identified a new function for AdrA in the transcriptional regulation of colanic acid and demonstrated that adrA and colanic acid biosynthesis are associated with S. enterica desiccation tolerance on the leaf surface. Finally, two PDEs with known roles in motility, STM1344 and STM1697, had competitive defects in the phyllosphere, suggesting that regulation of motility is crucial for S. enterica survival in this niche. Our results indicate that specific conditions influence the contribution of individual DGCs and PDEs to bacterial success, perhaps reflective of differential responses to environmental stimuli.
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Phytic Acid and Sodium Chloride Show Marked Synergistic Bactericidal Effects Against Non-adapted and Acid-adapted Escherichia coli O157:H7

The synergistic antimicrobial effects of phytic acid (PA), a natural extract from rice bran, plus sodium chloride (NaCl) against Escherichia coli O157:H7 were examined. Exposure to NaCl alone at concentrations up to 36% (w/w) for 5 min did not reduce bacterial populations. The bactericidal effects of PA alone were much greater than those of other organic acids (acetic, citric, lactic, and malic acids) under the same experimental conditions (p 7 log CFU/ml reduction). Flow cytometry confirmed that PA disrupted the cell membrane to a greater extent than other organic acids, although the cells remained viable. The combination of PA and NaCl induced complete disintegration of the cell membrane. By comparison, none of the other organic acids acted synergistically with NaCl; nor did NaCl/HCl solutions at the same pH values as the test PA + NaCl solutions. This result suggests that PA has great potential as an effective bacterial membrane permeabilizing agent, and show that the combination is a promising alternative to conventional chemical disinfectants. These findings provide new insight into the utility of natural compounds as novel antimicrobial agents and increase our understanding of the mechanism underlying the antibacterial activity of PA.
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Pectin and Xyloglucan Influence the Attachment of Salmonella enterica and Listeria monocytogenes to Bacterial Cellulose Derived Plant Cell Wall Models

Minimally processed fresh produce has been implicated as a major source of microbial foodborne pathogens globally. These pathogens must attach to the produce in order to be transmitted. Cut surfaces of produce that expose cell wall are particularly vulnerable. Little is known about the role that different structural components (cellulose, pectin and xyloglucan) of plant cell walls play in the attachment of bacterial foodborne pathogens. Using bacterial cellulose-derived plant cell wall models we show that the presence of pectin alone or xyloglucan alone affected the attachment of three Salmonella enterica (S. Enteritidis ATCC 13076, S. Typhimurium ATCC 14028 and S. enterica M4) and Listeria monocytogenes ATCC 7644. In addition we show this effect was further modulated in the presence of both polysaccharides. Assays using pairwise combinations of S. Typhimurium ATCC 14028 and L. monocytogenes ATCC 7644 showed that bacterial attachment to all plant cell wall models was dependent on the characteristics of the individual bacterial strains and not directly proportional to the initial concentration of the bacterial inoculum. This work shows that bacterial attachment was not directly determined by plant cell wall model or bacterial physicochemical properties. We suggest that attachment of the Salmonella strains may be influenced by the effect of these polysaccharides on the physical and structural properties of the plant cell wall model. Our findings improve understanding of how Salmonella enterica and Listeria monocytogenes attach to plant cell walls. This may aid in developing better ways to prevent attachment of these pathogens to these surfaces.
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Meta-analysis of the Effects of Sanitizing Treatments on Salmonella, Escherichia coli O157:H7, and Listeria monocytogenes Inactivation in Fresh Produce

Meta-analysis of the Effects of Sanitizing Treatments on Salmonella, Escherichia coli O157:H7, and Listeria monocytogenes Inactivation in Fresh Produce | Plant - Salmonella or E. coli Interactions | Scoop.it
The aim of this study was to perform a meta-analysis of the effects of sanitizing treatments of fresh produce on Salmonella spp., Escherichia coli O157:H7, and Listeria monocytogenes. From 55 primary studies found to report on such effects, 40 were selected based on specific criteria, leading to more than 1,000 data on mean log reductions of these three bacterial pathogens impairing the safety of fresh produce. Data were partitioned to build three meta-analytical models that could allow the assessment of differences in mean log reductions among pathogens, fresh produce, and sanitizers. Moderating variables assessed in the meta-analytical models included type of fresh produce, type of sanitizer, concentration, and treatment time and temperature. Further, a proposal was done to classify the sanitizers according to bactericidal efficacy by means of a meta-analytical dendrogram. The results indicated that both time and temperature significantly affected the mean log reductions of the sanitizing treatment (P < 0.0001). In general, sanitizer treatments led to lower mean log reductions when applied to leafy greens (for example, 0.68 log reductions [0.00 to 1.37] achieved in lettuce) compared to other, nonleafy vegetables (for example, 3.04 mean log reductions [2.32 to 3.76] obtained for carrots). Among the pathogens, E. coli O157:H7 was more resistant to ozone (1.6 mean log reductions), while L. monocytogenes and Salmonella presented high resistance to organic acids, such as citric acid, acetic acid, and lactic acid (∼3.0 mean log reductions). With regard to the sanitizers, it has been found that slightly acidic electrolyzed water, acidified sodium chlorite, and the gaseous chlorine dioxide clustered together, indicating that they possessed the strongest bactericidal effect. The results reported seem to be an important achievement for advancing the global understanding of the effectiveness of sanitizers for microbial safety of fresh produce.
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