The use of flagella and motility for plant colonization and fitness by different strains of the foodborne pathogen Listeria monocytogenes

EⅡB Mutation Reduces the Pathogenicity of Listeria monocytogenes by Negatively Regulating Biofilm Formation Ability, Infective Capacity, and Virulence Gene Expression

To explore the role of the membrane permease ⅡB (EⅡB) gene of Listeria pathogenicity island 4 (LIPI-4) in the virulence of Listeria monocytogenes, both an EⅡB deletion strain (∆EⅡB) and a complemented strain were constructed. In vitro experiments demonstrated that EⅡB deletion affected the biofilm formation ability of the wild-type strain (Lm928). Moreover, this deletion decreased the intracellular proliferation abilities of L. monocytogenes. Mice infected with ∆EⅡB survived longer and experienced less weight loss on days 1, 2, and 3 post-infection. The bacterial load in the liver tissue of ∆EⅡB-infected mice was significantly reduced, and a considerable decrease in the blood levels of inflammatory cytokines IL-β, IL-6, IL-10, and TNF-α were observed. Following EⅡB deletion, 65% (13/20) of genes were downregulated, 25% (5/20) were upregulated, and 10% (2/20) showed no change. These findings suggest that EⅡB deletion may reduce both the in vivo and in vitro virulence levels as well as the biofilm formation ability of Lm928 by downregulating the transcription levels of genes associated with virulence and biofilm formation. These findings provide a foundation for further examining the pathogenic mechanisms of LIPI-4 and EⅡB in L. monocytogenes.

Growth, biofilm formation, and motility of Listeria monocytogenes strains isolated from food and clinical samples located in Shanghai (China)

Listeria monocytogenes is a common foodborne pathogen that frequently causes global outbreaks. In this study, the growth characteristics, biofilm formation ability, motility ability and whole genome of 26 L. monocytogenes strains isolated from food and clinical samples in Shanghai (China) from 2020 to 2022 were analyzed. There are significant differences among isolates in terms of growth, biofilm formation, motility, and gene expression. Compared with other sequence type (ST) types, ST1930 type exhibited a significantly higher maximum growth rate, the ST8 type demonstrated a stronger biofilm formation ability, and the ST121 type displayed greater motility ability. Furthermore, ST121 exhibited significantly high mRNA expression levels compared with other ST types in virulence genes mpl, fbpA and fbpB, the quorum sensing gene luxS, starvation response regulation gene relA, and biofilm adhesion related gene bapL. Whole-genome sequencing (WGS) analyses indicated the isolates of lineage I were mostly derived from clinical, and the isolates of lineage II were mostly derived from food. The motility ability, along with the expression of genes associated with motility (motA and motB), exhibited a significantly higher level in lineage II compared with lineage I. The isolates from food exhibited significantly higher motility ability compared with isolates from clinical. By integrating growth, biofilm formation, motility phenotype with molecular and genotyping information, it is possible to enhance comprehension of the association between genes associated with these characteristics in L. monocytogenes.

Colonization of Raphanus sativus by human pathogenic microorganisms

Contamination of vegetables with human pathogenic microorganisms (HPMOs) is considered one of the most important problems in the food industry, as current nutritional guidelines include increased consumption of raw or minimally processed organic vegetables due to healthy lifestyle promotion. Vegetables are known to be potential vehicles for HPMOs and sources of disease outbreaks. In this study, we tested the susceptibility of radish (Raphanus sativus) to colonization by different HPMOs, including Escherichia coli PCM 2561, Salmonella enterica subsp. enterica PCM 2565, Listeria monocytogenes PCM 2191 and Bacillus cereus PCM 1948. We hypothesized that host plant roots containing bactericidal compounds are less prone to HPMO colonization than shoots and leaves. We also determined the effect of selected pathogens on radish growth to check host plant-microbe interactions. We found that one-week-old radish is susceptible to colonization by selected HPMOs, as the presence of the tested HPMOs was demonstrated in all organs of R. sativus. The differences were noticed 2 weeks after inoculation because B. cereus was most abundant in roots (log10 CFU - 2.54), S. enterica was observed exclusively in stems (log10 CFU - 3.15), and L. monocytogenes and E. coli were most abundant in leaves (log10 CFU - 4.80 and 3.23, respectively). The results suggest that E. coli and L. monocytogenes show a higher ability to colonize and move across the plant than B. cereus and S. enterica. Based on fluorescence in situ hybridization (FISH) and confocal laser scanning microscopy (CLSM) approach HPMOs were detected in extracellular matrix and in some individual cells of all analyzed organs. The presence of pathogens adversely affected the growth parameters of one-week-old R. sativus, especially leaf and stem fresh weight (decreased by 47-66 and 17-57%, respectively). In two-week-old plants, no reduction in plant biomass development was noted. This observation may result from plant adaptation to biotic stress caused by the presence of HPMOs, but confirmation of this assumption is needed. Among the investigated HPMOs, L. monocytogenes turned out to be the pathogen that most intensively colonized the aboveground part of R. sativus and at the same time negatively affected the largest number of radish growth parameters.

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.

Flagellar Genes Are Associated with the Colonization Persistence Phenotype of the Drosophila melanogaster Microbiota

In this work, we use Drosophila melanogaster as a model to identify bacterial genes necessary for bacteria to colonize their hosts independent of the bulk flow of diet. Early work on this model system established that dietary replenishment drives the composition of the D. melanogaster gut microbiota, and subsequent research has shown that some bacterial strains can stably colonize, or persist within, the fly independent of dietary replenishment. Here, we reveal transposon insertions in specific bacterial genes that influence the bacterial colonization persistence phenotype by using a gene association approach. We initially established that different bacterial strains persist at various levels, independent of dietary replenishment. We then repeated the analysis with an expanded panel of bacterial strains and performed a metagenome-wide association (MGWA) study to identify distinct bacterial genes that are significantly correlated with the level of colonization by persistent bacterial strains. Based on the MGWA study, we tested if 44 bacterial transposon insertion mutants from 6 gene categories affect bacterial persistence within the flies. We identified that transposon insertions in four flagellar genes, one urea carboxylase gene, one phosphatidylinositol gene, one bacterial secretion gene, and one antimicrobial peptide (AMP) resistance gene each significantly influenced the colonization of D. melanogaster by an Acetobacter fabarum strain. Follow-up experiments revealed that each flagellar mutant was nonmotile, even though the wild-type strain was motile. Taken together, these results reveal that transposon insertions in specific bacterial genes, including motility genes, are necessary for at least one member of the fly microbiota to persistently colonize the fly. IMPORTANCE Despite the growing body of research on the microbiota, the mechanisms by which the microbiota colonizes a host can still be further elucidated. This study identifies bacterial genes that are associated with the colonization persistence phenotype of the microbiota in Drosophila melanogaster, which reveals specific bacterial factors that influence the establishment of the microbiota within its host. The identification of specific genes that affect persistence can help inform how the microbiota colonizes a host. Furthermore, a deeper understanding of the genetic mechanisms of the establishment of the microbiota could aid in the further development of the Drosophila microbiota as a model for microbiome research.

The Listeria monocytogenes exopolysaccharide significantly enhances colonization and survival on fresh produce

Fresh produce contaminated with Listeria monocytogenes has caused major listeriosis outbreaks in the last decades. Our knowledge about components of the listerial biofilms formed on fresh produce and their roles in causing foodborne illness remains incomplete. Here, we investigated, for the first time, the role of the listerial Pss exopolysaccharide (EPS) in plant surface colonization and stress tolerance. Pss is the main component of L. monocytogenes biofilms synthesized at elevated levels of the second messenger c-di-GMP. We developed a new biofilm model, whereby L. monocytogenes EGD-e and its derivatives are grown in the liquid minimal medium in the presence of pieces of wood or fresh produce. After 48-h incubation, the numbers of colony forming units of the Pss-synthesizing strain on pieces of wood, cantaloupe, celery and mixed salads were 2-12-fold higher, compared to the wild-type strain. Colonization of manmade materials, metals and plastics, was largely unaffected by the presence of Pss. The biofilms formed by the EPS-synthesizing strain on cantaloupe rind were 6-16-fold more tolerant of desiccation, which resembles conditions of whole cantaloupe storage and transportation. Further, listeria in the EPS-biofilms survived exposure to low pH, a condition encountered by bacteria on the contaminated produce during passage through the stomach, by 11-116-fold better than the wild-type strain. We surmise that L. monocytogenes strains synthesizing Pss EPS have an enormous, 10²-10⁴-fold, advantage over the non-synthesizing strains in colonizing fresh produce, surviving during storage and reaching small intestines of consumers where they may cause disease. The magnitude of the EPS effect calls for better understanding of factors inducing Pss synthesis and suggests that prevention of listerial EPS-biofilms may significantly enhance fresh produce safety.

Listeria motility increases the efficiency of epithelial invasion during intestinal infection

Listeria monocytogenes (Lm) is a food-borne pathogen that causes severe bacterial gastroenteritis, with high rates of hospitalization and mortality. Lm is ubiquitous in soil, water and livestock, and can survive and proliferate at low temperatures. Following oral ingestion of contaminated food, Lm crosses the epithelium through intestinal goblet cells in a mechanism mediated by Lm InlA binding host E-cadherin. Importantly, human infections typically occur with Lm growing at or below room temperature, which is flagellated and motile. Even though many important human bacterial pathogens are flagellated, little is known regarding the effect of Lm motility on invasion and immune evasion. Here, we used complementary imaging and computer modeling approaches to test the hypothesis that bacterial motility helps Lm locate and engage target cells permissive for invasion. Imaging explanted mouse and human intestine, we showed that Lm grown at room temperature uses motility to scan the epithelial surface and preferentially attach to target cells. Furthermore, we integrated quantitative parameters from our imaging experiments to construct a versatile "layered" cellular Potts model (L-CPM) that simulates host-pathogen dynamics. Simulated data are consistent with the hypothesis that bacterial motility enhances invasion by allowing bacteria to search the epithelial surface for their preferred invasion targets. Indeed, our model consistently predicts that motile bacteria invade twice as efficiently over the first hour of infection. We also examined how bacterial motility affected interactions with host cellular immunity. In a mouse model of persistent infection, we found that neutrophils migrated to the apical surface of the epithelium 5 hours post infection and interacted with Lm. Yet in contrast to the view that neutrophils "hunt" for bacteria, we found that these interactions were driven by motility of Lm-which moved at least ~50x faster than neutrophils. Furthermore, our L-CPM predicts that motile bacteria maintain their invasion advantage even in the presence of host phagocytes, with the balance between invasion and phagocytosis governed almost entirely by bacterial motility. In conclusion, our simulations provide insight into host pathogen interaction dynamics at the intestinal epithelial barrier early during infection.

Differential effects of magnesium, calcium, and sodium on Listeria monocytogenes biofilm formation

Listeria monocytogenes is a gram-positive foodborne pathogen that causes outbreaks of listeriosis associated with a diverse range of foods. L. monocytogenes forms biofilms as a strategy to enhance its survival in the environment. These biofilms then provide a source of contamination in processing plant environments. Cations like magnesium, calcium, and sodium are commonly found in the environment and are important to bacteria to maintain their homeostasis. It is, therefore, valuable to understand the relationship between these cations and biofilm formation. In this study, four isolates of L. monocytogenes from seafood processing environments were used to investigate the influence of magnesium, calcium, and sodium (1, 10, and 50 mM) on biofilms. The isolates selected were defined as being either a low biofilm former, a high biofilm former, an outbreak isolate, and a persistent isolate from the seafood industry. The study showed that the divalent cations magnesium and calcium increased biofilm formation compared with the monovalent cation, sodium. Fifty mM concentrations of the divalent cations significantly enhanced biofilm formation. The cations did not have a significant effect on the initial stages of biofilm formation but appeared to influence the later stages of biofilm development.

Listeria monocytogenes - How This Pathogen Survives in Food-Production Environments?

The foodborne pathogen Listeria monocytogenes is the causative agent of human listeriosis, a severe disease, especially dangerous for the elderly, pregnant women, and newborns. Although this infection is comparatively rare, it is often associated with a significant mortality rate of 20-30% worldwide. Therefore, this microorganism has an important impact on food safety. L. monocytogenes can adapt, survive and even grow over a wide range of food production environmental stress conditions such as temperatures, low and high pH, high salt concentration, ultraviolet lights, presence of biocides and heavy metals. Furthermore, this bacterium is also able to form biofilm structures on a variety of surfaces in food production environments which makes it difficult to remove and allows it to persist for a long time. This increases the risk of contamination of food production facilities and finally foods. The present review focuses on the key issues related to the molecular mechanisms of the pathogen survival and adaptation to adverse environmental conditions. Knowledge and understanding of the L. monocytogenes adaptation approaches to environmental stress factors will have a significant influence on the development of new, efficient, and cost-effective methods of the pathogen control in the food industry, which is critical to ensure food production safety.

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

ABSTRACT

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

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Lipopolysaccharide and flagellin of Azospirillum brasilense Sp7 influence callus morphogenesis and plant regeneration in wheat

In vitro somatic callus culturing is used widely in plant biotechnology, but its effectiveness depends largely on the donor plant genotype. Bacteria or components of their cells are rarely used to activate morphogenesis. In this work, inoculation of explants from immature wheat (Triticum aestivum L.) embryos with a suspension of living cells of the bacterium Azospirillum brasilense Sp7 resulted in callus death after 7 days of growth, in contrast to explant treatment with a suspension of heat-killed whole cells of Sp7. The experiments used two wheat lines, LRht-B1a and LRht-B1c, which differ in morphogenic activity. Growing calluses with the lipopolysaccharide of A. brasilense Sp7 increased the yield of regenerated plants 2- to 3.5-fold in both lines. This increase was through the activation of regenerant formation from morphogenic calluses. We have demonstrated for the first time the effects of bacterial flagellin on plant tissue culture. The polar-flagellum flagellin of A. brasilense Sp7 leveled the genotypic differences in the morphogenic ability of callus tissue. Specifically, it increased the yield of morphogenic calluses in the weakly morphogenic line LRht-B1a to the yield value in the highly morphogenic line LRht-B1c but lowered the yield of regenerants in the highly morphogenic line LRht-B1c to the yield value in the weakly morphogenic line LRht-B1a. Thus, bacterial lipopolysaccharides and flagellins can be used to regulate the formation of morphogenic calluses and regenerants in plant tissue culturing in vitro.

Rhizobacteria Impact Colonization of Listeria monocytogenes on Arabidopsis thaliana Roots

In spite of its relevance as a foodborne pathogen, we have limited knowledge about Listeria monocytogenes in the environment. L. monocytogenes outbreaks have been linked to fruits and vegetables; thus, a better understanding of the factors influencing its ability to colonize plants is important. We tested how environmental factors and other soil- and plant-associated bacteria influenced L. monocytogenes' ability to colonize plant roots using Arabidopsis thaliana seedlings in a hydroponic growth system. We determined that the successful root colonization of L. monocytogenes 10403S was modestly but significantly enhanced by the bacterium being pregrown at higher temperatures, and this effect was independent of the biofilm and virulence regulator PrfA. We tested 14 rhizosphere-derived bacteria for their impact on L. monocytogenes 10403S, identifying one that enhanced and 10 that inhibited the association of 10403S with plant roots. We also characterized the outcomes of these interactions under both coinoculation and invasion conditions. We characterized the physical requirements of five of these rhizobacteria to impact the association of L. monocytogenes 10403S with roots, visualizing one of these interactions by microscopy. Furthermore, we determined that two rhizobacteria (one an inhibitor, the other an enhancer of 10403S root association) were able to similarly impact 10 different L. monocytogenes strains, indicating that the effects of these rhizobacteria on L. monocytogenes are not strain specific. Taken together, our results advance our understanding of the parameters that affect L. monocytogenes plant root colonization, knowledge that may enable us to deter its association with and, thus, downstream contamination of, food crops. IMPORTANCE Listeria monocytogenes is ubiquitous in the environment, being found in or on soil, water, plants, and wildlife. However, little is known about the requirements for L. monocytogenes' existence in these settings. Recent L. monocytogenes outbreaks have been associated with contaminated produce; thus, we used a plant colonization model to investigate factors that alter L. monocytogenes' ability to colonize plant roots. We show that L. monocytogenes colonization of roots was enhanced when grown at higher temperatures prior to inoculation but did not require a known regulator of virulence and biofilm formation. Additionally, we identified several rhizobacteria that altered the ability of 11 different strains of L. monocytogenes to colonize plant roots. Understanding the factors that impact L. monocytogenes physiology and growth will be crucial for finding mechanisms (whether chemical or microbial) that enable its removal from plant surfaces to reduce L. monocytogenes contamination of produce and eliminate foodborne illness.

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.

Colonisation dynamics of Listeria monocytogenes strains isolated from food production environments

Listeria monocytogenes is a ubiquitous bacterium capable of colonising and persisting within food production environments (FPEs) for many years, even decades. This ability to colonise, survive and persist within the FPEs can result in food product cross-contamination, including vulnerable products such as ready to eat food items. Various environmental and genetic elements are purported to be involved, with the ability to form biofilms being an important factor. In this study we examined various mechanisms which can influence colonisation in FPEs. The ability of isolates (n = 52) to attach and grow in biofilm was assessed, distinguishing slower biofilm formers from isolates forming biofilm more rapidly. These isolates were further assessed to determine if growth rate, exopolymeric substance production and/or the agr signalling propeptide influenced these dynamics and could promote persistence in conditions reflective of FPE. Despite no strong association with the above factors to a rapid colonisation phenotype, the global transcriptome suggested transport, energy production and metabolism genes were widely upregulated during the initial colonisation stages under nutrient limited conditions. However, the upregulation of the metabolism systems varied between isolates supporting the idea that L. monocytogenes ability to colonise the FPEs is strain-specific.

Growth and Survival of Attached Listeria on Lettuce and Stainless Steel Varies by Strain and Surface Type

ABSTRACT

The foodborne pathogen Listeria monocytogenes lives as a saprophyte in nature and can adhere to and grow on surfaces as diverse as leaves, sediment, and stainless steel. To discern the mechanisms used by L. monocytogenes for attachment and growth on various surfaces, we studied interactions between the pathogen on lettuce and stainless steel. A panel of 24 strains (23 L. monocytogenes and 1 Listeria innocua) were screened for attachment and growth on lettuce at 4 and 25°C and on stainless steel at 10 and 37°C. Overnight growth of attached cells resulted in a 0- to 3-log increase on lettuce, depending on the strain and the temperature. Among the worst-performing strains on lettuce were two from a large cantaloupe outbreak, indicating that factors important for interactions with cantaloupe may be different from those required on lettuce tissue. Strains that grew the best on lettuce belonged to serotypes 1/2a, 1/2b, and 4b and were from cheese, potatoes, and water-sediment near produce fields. Confocal microscopy of L. monocytogenes tagged with constitutively expressed green fluorescent protein indicated associations with the cut edges and veins of lettuce leaves. On stainless steel coupons, there was a 5- to 7-log increase at 10°C after 7 days and a 4- to 7-log increase at 37°C after 40 h. Statistically, surface growth on stainless steel was better for serotype 1/2a than for serotype 4b strains, even though certain serotype 4b strains grew well on the coupons. The latter included strains that originated from produce and water-sediment. Some strains were fit in both environments, whereas others showed variability between the two different surfaces. Further analysis of these strains should reveal molecular factors needed for adherence and surface growth of L. monocytogenes on different biotic and abiotic surfaces.

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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.

Food Microstructure and Fat Content Affect Growth Morphology, Growth Kinetics, and Preferred Phase for Cell Growth of Listeria monocytogenes in Fish-Based Model Systems

Food microstructure significantly affects microbial growth dynamics, but knowledge concerning the exact influencing mechanisms at a microscopic scale is limited. The food microstructural influence on Listeria monocytogenes (green fluorescent protein strain) growth at 10°C in fish-based food model systems was investigated by confocal laser scanning microscopy. The model systems had different microstructures, i.e., liquid, xanthan (high-viscosity liquid), aqueous gel, and emulsion and gelled emulsion systems varying in fat content. Bacteria grew as single cells, small aggregates, and microcolonies of different sizes (based on colony radii [size I, 1.5 to 5.0 μm; size II, 5.0 to 10.0 μm; size III, 10.0 to 15.0 μm; and size IV, ≥15 μm]). In the liquid, small aggregates and size I microcolonies were predominantly present, while size II and III microcolonies were predominant in the xanthan and aqueous gel. Cells in the emulsions and gelled emulsions grew in the aqueous phase and on the fat-water interface. A microbial adhesion to solvent assay demonstrated limited bacterial nonpolar solvent affinities, implying that this behavior was probably not caused by cell surface hydrophobicity. In systems containing 1 and 5% fat, the largest cell volume was mainly represented by size I and II microcolonies, while at 10 and 20% fat a few size IV microcolonies comprised nearly the total cell volume. Microscopic results (concerning, e.g., growth morphology, microcolony size, intercolony distances, and the preferred phase for growth) were related to previously obtained macroscopic growth dynamics in the model systems for an L. monocytogenes strain cocktail, leading to more substantiated explanations for the influence of food microstructural aspects on lag phase duration and growth rate.IMPORTANCEListeria monocytogenes is one of the most hazardous foodborne pathogens due to the high fatality rate of the disease (i.e., listeriosis). In this study, the growth behavior of L. monocytogenes was investigated at a microscopic scale in food model systems that mimic processed fish products (e.g., fish paté and fish soup), and the results were related to macroscopic growth parameters. Many studies have previously focused on the food microstructural influence on microbial growth. The novelty of this work lies in (i) the microscopic investigation of products with a complex composition and/or structure using confocal laser scanning microscopy and (ii) the direct link to the macroscopic level. Growth behavior (i.e., concerning bacterial growth morphology and preferred phase for growth) was more complex than assumed in common macroscopic studies. Consequently, the effectiveness of industrial antimicrobial food preservation technologies (e.g., thermal processing) might be overestimated for certain products, which may have critical food safety implications.

Antioxidant Properties and Antibacterial Effects of Eucalyptus camaldulensis Ethanolic Leaf Extract on Biofilm Formation, Motility, Hemolysin Production, and Cell Membrane of the Foodborne Pathogen Listeria monocytogenes

Consumer concerns toward chemical preservatives have resulted in increased search for healthy green alternative. In this study, the antioxidant activity and antibacterial effects of Eucalyptus camaldulensis ethanolic leaf extract against Listeria monocytogenes, a serious foodborne pathogen, was evaluated. Total phenolic and flavonoid contents of the extract were 11.10 mg garlic acid equivalent/mg extract and 15.05 mg quercetin equivalent/mg extract, respectively. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration of the extract was 64-128 μg/mL and 256-512 μg/mL, respectively. Time-kill assay revealed growth inhibitory effects after 4-h treatment of the bacteria with the extract. A reduction of ≈2-3 log colony-forming units per milliliter was observed against the tested food and environmental isolates after challenging the pathogens with the extract at MIC for 6 h. Sub-MICs of the extract significantly inhibited motility and listeriolysin O production up to 80%, with 60% inhibition of biofilm formation (p < 0.05). Antioxidant assay revealed free radical scavenging activity with 50% inhibitory concentration (IC₅₀) of 57.07 μg/mL for 2,2-diphenyl-1-picrylhydrazyl and 29.01 μg/mL for ABTS [2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)] assay. Ferric reducing antioxidant power assay further showed a total antioxidant power equivalent to 92.93 μM ascorbic acid equivalent/mg extract. As the extract exhibited profound antilisterial activity and good radical scavenging ability, it might serve as a potential alternative source of biopreservative agent against L. monocytogenes.

Validation of an ampicillin selection protocol to enrich for mutants of Listeria monocytogenes unable to replicate on fresh produce

Several outbreaks of listeriosis have implicated fresh produce but genetic factors required for growth of Listeria monocytogenes on produce remain poorly characterized. Based on the fact that β-lactam antibiotics only kill bacterial cells that are growing, we hypothesized that ampicillin selection can enrich for L. monocytogenes mutants unable to grow on produce. For validation, we examined relative recovery of L. monocytogenes strain 2011L-2858 and its cold-sensitive mutant L1E4 following inoculation of cantaloupe rind fragments with 1:1 mixture of the strains and incubation at 4°C with or without ampicillin. Listeria monocytogenes from rind fragments inoculated with the mixed cultures and incubated in the presence of ampicillin were used to inoculate fresh rind fragments for a second round of enrichment. In the presence of ampicillin, the proportion of L1E4 increased from 55% on day 0 to 78% on day 14, with higher recovery (85% after 14 days) in the second round of enrichment. These data suggested that L1E4 was enriched on cantaloupe rind fragments while growing cells of the wildtype were killed by ampicillin. Application of this protocol to transposon mutant libraries from three L. monocytogenes strains yielded several mutants unable to grow on cantaloupe. Thus, ampicillin selection can facilitate discovery of genes essential for growth of L. monocytogenes on fresh produce.

RNA Helicase Mediates Competitive Fitness of Listeria monocytogenes on the Surface of Cantaloupe

Listeria monocytogenes is a foodborne pathogen that is implicated in numerous outbreaks of disease (listeriosis) via fresh produce. The genetic features of L. monocytogenes that allow adherence and growth on produce remain largely uncharacterized. In this study, two non-motile transposon mutants were characterized for attachment, growth, and survival on the surface of cantaloupe rind. One of the mutants, L1E4, harbored a single transposon insertion in a DEAD-box RNA helicase gene (lmo0866 homolog), while the other, M1A5, harbored an insertion in a gene from a flagellum biosynthesis and chemotaxis gene cluster (lmo0694 homolog). When inoculated alone, neither mutant was significantly impaired in growth or survival on the surface of cantaloupe at either 25 or 37 °C. However, when co-inoculated with the wildtype parental strain, the RNA helicase mutant L1E4 had a clear competitive disadvantage, while the relative fitness of M1A5 was not noticeably impacted. Genetic complementation of L1E4 with the intact RNA helicase gene restored relative fitness on cantaloupe. The findings suggest that the DEAD-box RNA helicase encoded by the lmo0866 homolog is critical for relative fitness of L. monocytogenes on cantaloupe. Mutant L1E4 was pleiotropic, being not only non-motile but also cold-sensitive and with reduced hemolytic activity, warranting further studies to elucidate the role of this helicase in the competitive fitness of L. monocytogenes on produce.

Listeria monocytogenes Internalizes in Romaine Lettuce Grown in Greenhouse Conditions

Listeria monocytogenes has been implicated in a number of outbreaks involving fresh produce, including an outbreak in 2016 resulting from contaminated packaged salads. The persistence and internalization potential of L. monocytogenes in romaine lettuce was evaluated, and the persistence of two L. monocytogenes strains was assessed on three romaine lettuce cultivars. Seeds were germinated, and plants grown in three soil types (i.e., standard potting mix, autoclaved potting mix, and top soil) and sterile soft-top agar for up to 21 days. Average CFU per gram of L. monocytogenes on seeds and plants was calculated from five replicates per harvest day. Up to 8.2 log CFU/g L. monocytogenes persisted on romaine lettuce plants (Braveheart cultivar) grown in soft-top agar, while those grown in commercial potting mix (initial soil aerobic plate count of 4.0 × 104 CFU/g) had a final concentration of 5.4 log CFU/g, and autoclaved commercial potting mix had a final concentration of 3.8 ± 0.2 log CFU/g after a 21-day period. Pathogen levels dropped below the limit of detection (2 log CFU/g) by day 18 in 75% topsoil (initial soil aerobic plate count of 4.0 × 101 CFU/g); this did not occur in sterile media. Although L. monocytogenes strain differences and presence of a clay coating on seeds did not affect persistence, differences were observed in L. monocytogenes growth and survival among cultivars. To assess internalization, seeds were inoculated with L. monocytogenes expressing green fluorescent protein. Three plants were fixed, paraffin embedded, and sectioned; localization was studied by using standard immunohistochemistry techniques. A total of 539 internalized L. monocytogenes cells were visualized among three 20-day seedlings. L. monocytogenes cells were located in all major tissue types (pith followed by cortex, xylem, phloem, and epidermis). The presence of L. monocytogenes in the plant vasculature suggests potential for transport throughout the plant into edible tissue.

FleQ coordinates flagellum-dependent and -independent motilities in Pseudomonas syringae pv. tomato DC3000

Motility plays an essential role in bacterial fitness and colonization in the plant environment, since it favors nutrient acquisition and avoidance of toxic substances, successful competition with other microorganisms, the ability to locate the preferred hosts, access to optimal sites within them, and dispersal in the environment during the course of transmission. In this work, we have observed that the mutation of the flagellar master regulatory gene, fleQ, alters bacterial surface motility and biosurfactant production, uncovering a new type of motility for Pseudomonas syringae pv. tomato DC3000 on semisolid surfaces. We present evidence that P. syringae pv. tomato DC3000 moves over semisolid surfaces by using at least two different types of motility, namely, swarming, which depends on the presence of flagella and syringafactin, a biosurfactant produced by this strain, and a flagellum-independent surface spreading or sliding, which also requires syringafactin. We also show that FleQ activates flagellum synthesis and negatively regulates syringafactin production in P. syringae pv. tomato DC3000. Finally, it was surprising to observe that mutants lacking flagella or syringafactin were as virulent as the wild type, and only the simultaneous loss of both flagella and syringafactin impairs the ability of P. syringae pv. tomato DC3000 to colonize tomato host plants and cause disease.

Genes Associated with Desiccation and Osmotic Stress in Listeria monocytogenes as Revealed by Insertional Mutagenesis

Listeria monocytogenes is a foodborne pathogen whose survival in food processing environments may be associated with its tolerance to desiccation. To probe the molecular mechanisms used by this bacterium to adapt to desiccation stress, a transposon library of 11,700 L. monocytogenes mutants was screened, using a microplate assay, for strains displaying increased or decreased desiccation survival (43% relative humidity, 15°C) in tryptic soy broth (TSB). The desiccation phenotypes of selected mutants were subsequently assessed on food-grade stainless steel (SS) coupons in TSB plus 1% glucose (TSB-glu). Single transposon insertions in mutants exhibiting a change in desiccation survival of >0.5 log CFU/cm(2) relative to that of the wild type were determined by sequencing arbitrary PCR products. Strain morphology, motility, and osmotic stress survival (in TSB-glu plus 20% NaCl) were also analyzed. The initial screen selected 129 desiccation-sensitive (DS) and 61 desiccation-tolerant (DT) mutants, out of which secondary screening on SS confirmed 15 DT and 15 DS mutants. Among the DT mutants, seven immotile and flagellum-less strains contained transposons in genes involved in flagellum biosynthesis (fliP, flhB, flgD, flgL) and motor control (motB, fliM, fliY), while others harbored transposons in genes involved in membrane lipid biosynthesis, energy production, potassium uptake, and virulence. The genes that were interrupted in the 15 DS mutants included those involved in energy production, membrane transport, protein metabolism, lipid biosynthesis, oxidative damage control, and putative virulence. Five DT and 14 DS mutants also demonstrated similar significantly (P < 0.05) different survival relative to that of the wild type when exposed to osmotic stress, demonstrating that some genes likely have similar roles in allowing the organism to survive the two water stresses.

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.

PrfA-like transcription factor gene lmo0753 contributes to L-rhamnose utilization in Listeria monocytogenes strains associated with human food-borne infections

Listeria monocytogenes is a food-borne bacterial pathogen and the causative agent of human and animal listeriosis. Among the three major genetic lineages of L. monocytogenes (i.e., LI, LII, and LIII), LI and LII are predominantly associated with food-borne listeriosis outbreaks, whereas LIII is rarely implicated in human infections. In a previous study, we identified a Crp/Fnr family transcription factor gene, lmo0753, that was highly specific to outbreak-associated LI and LII but absent from LIII. Lmo0753 shares two conserved functional domains, including a DNA binding domain, with the well-characterized master virulence regulator PrfA in L. monocytogenes. In this study, we constructed lmo0753 deletion and complementation mutants in two fully sequenced L. monocytogenes LII strains, 10403S and EGDe, and compared the flagellar motility, phospholipase C production, hemolysis, and intracellular growth of the mutants and their respective wild types. Our results suggested that lmo0753 plays a role in hemolytic activity in both EGDe and 10403S. More interestingly, we found that deletion of lmo0753 led to the loss of l-rhamnose utilization in EGDe, but not in 10403S. RNA-seq analysis of EGDe Δ0753 incubated in phenol red medium containing l-rhamnose as the sole carbon source revealed that 126 (4.5%) and 546 (19.5%) out of 2,798 genes in the EGDe genome were up- and downregulated more than 2-fold, respectively, compared to the wild-type strain. Genes related to biotin biosynthesis, general stress response, and rhamnose metabolism were shown to be differentially regulated. Findings from this study collectively suggested varied functional roles of lmo0753 in different LII L. monocytogenes strain backgrounds associated with human listeriosis outbreaks.

Proteomic analysis of Herbaspirillum seropedicae cultivated in the presence of sugar cane extract

Bacterial endophytes of the genus Herbaspirillum colonize sugar cane and can promote plant growth. The molecular mechanisms that mediate plant- H. seropedicae interaction are poorly understood. In this work, we used 2D-PAGE electrophoresis to identify H. seropedicae proteins differentially expressed at the log growth phase in the presence of sugar cane extract. The differentially expressed proteins were validated by RT qPCR. A total of 16 differential spots (1 exclusively expressed, 7 absent, 5 up- and 3 down-regulated) in the presence of 5% sugar cane extract were identified; thus the host extract is able to induce and repress specific genes of H. seropedicae. The differentially expressed proteins suggest that exposure to sugar cane extract induced metabolic changes and adaptations in H. seropedicae presumably in preparation to establish interaction with the plant.

The importance of motility and chemotaxis for extra-animal survival of Salmonella enterica serovar Typhimurium and Dublin

AIMS

This study investigated the importance of flagella and motility of Salmonella enterica serovar Typhimurium and Dublin in models of extra-animal survival.

METHODS AND RESULTS

The study was performed using transposon mutants in flagella genes fliC and fljB and in chemotaxis genes cheA, cheB and cheR. Flagella and chemotaxis were found to be of minor importance for attachment to plant leaves, survival in liquid manure and interaction with the nematode C. elegans, while differences were observed between the fliC mutant and the wild-type strain of S. Dublin in interactions with amoebae.

CONCLUSIONS

The study shows that flagella and chemotaxis play a minor role in extra-animal survival of these two serovars of Salmonella under the conditions tested.

SIGNIFICANCE AND IMPACT OF THE STUDY

Extra-animal survival is important in the full infection cycle for zoonotic salmonellae. Such serovars are motile. Even though the current study was only based on the characterization of two serovars, it strongly suggests that motility and chemotaxis are of minor importance during the spread of Salmonella from one animal to the next through the external environment.

Role of flhA and motA in growth of Listeria monocytogenes at low temperatures

While temperature-dependent induction of flagella is a well-characterized phenomenon in Listeria monocytogenes, the essentiality of increased flagellum production during growth at low temperatures remains unclear. To study this relationship, we compared the relative expression levels of two motility genes, flhA and motA, at 3°C, 25°C and 37°C in L. monocytogenes strain EGD-e by using qRT-PCR, and compared the growth curves, motility, and flagellation between the wild-type and flhA and motA deletion mutants. The relative expression levels of flhA and motA at 3°C were significantly higher than at 37°C (p<0.01). At 3°C, the level of flhA transcripts was also significantly higher than at 25°C (p<0.01). Growth curve analysis showed that at 3°C both the growth rates and maximum optical densities of ΔflhA and ΔmotA strains at 600 nm were significantly lower than those of the wild-type (p<0.001), while no significant differences were observed between the wild-type and the mutants at 37°C, and 25°C. Mutant strains ΔflhA and ΔmotA were nonmotile at all three temperatures. At 25°C, the number of flagellated cells of ΔmotA was notably reduced compared with the wild-type, whereas ΔflhA appeared nonflagellated at all temperatures. The results suggest that flhA and motA play a role in the cold tolerance of L. monocytogenes strain EGD-e, and that motile flagella may be needed for optimal cold stress response of L. monocytogenes.

The Sigma B operon is a determinant of fitness for a Listeria monocytogenes serotype 4b strain in soil

In nature the foodborne pathogen Listeria monocytogenes lives as a saprophyte where it can contaminate preharvest produce. This environment can present many stresses such as ultraviolet light, variations in temperature and humidity, and oxidative stress from growing plant matter in the soil. The alternative sigma factor Sigma B, encoded by sigB, controls the response to most stresses in L. monocytogenes. Fitness in soil and on radishes sown and grown in contaminated soil was measured in a wild-type and an isogenic sigB operon mutant strain to determine if the sigma factor was necessary for life in these niches. Levels of wild-type and mutant strains were monitored in contaminated soil over the course of radish gestation from seed to mature tuber, and levels on mature radishes were determined. The wild-type strain was able to survive in soil over the 4 weeks of the experiment at levels of 4-7 log CFU/g soil, and the levels of the sigB mutant were reduced by 1-2 log from the wild type. The mutant showed reduced levels in soil by 6 h after inoculation, which was partially recovered when the mutant was complemented, and stayed at a reduced level over the next 4 weeks. Upon harvest, 3-4 log CFU/g of wild-type L. monocytogenes was detected on radish surfaces, and the bacteria could not be washed off under running water. On mature radishes populations of the mutant strain were 1-2 log CFU/g lower than the wild type. The levels on mature radishes reflected the levels in the soil at 4 weeks. The conclusions are that the Sigma B operon is necessary for initial adaptation to the soil environment, and plays a role in maintaining the population, but does not play a role in attachment or colonization of the radish.

Changes in intestinal Toll-like receptors and cytokines precede histological injury in a rat model of necrotizing enterocolitis

It is unclear whether the broad inflammatory response shown in neonatal necrotizing enterocolitis (NEC) is the cause or the effect of tissue injury. Toll-like receptors (TLRs) on intestinal dendritic, mononuclear, and epithelial cells recognize bacterial ligands and damaged tissues, thus activating the inflammatory response. The present study aimed to determine whether active TLR signaling would precede histological injury in NEC. Newborn rat pups were divided into four groups: dam fed, dam fed-hypoxic, formula fed, and formula fed-hypoxic (NEC). The ileal tissues were evaluated for NEC scores at 24, 48, 72, and 120 h. Quantitative real-time reverse transcription-polymerase chain reaction and immunohistochemistry were used to measure and localize intestinal TLRs. Cytokines were assessed by a multispot cytokine array. Among the four groups, ileal injury was seen only after 72 h of formula feeding and hypoxia. We found selective induction of mRNA levels in NEC compared with dam-fed controls for TLR2 > TLR4 > TLR1 = TLR3, TLR7, and TLR9 > TLR6 (P < 0.01); TLR5 was downregulated (P < 0.01). All TLR changes started at 48 h, before any histological evidence of NEC. Both Th1-type cytokines (IFN-gamma, IL-1beta, TNF-alpha, and KC/GRO) and Th2-type cytokines (IL-4, IL-5 and IL-13) were significantly increased in NEC but also in nondamaged formula-fed rat ileum. In conclusion, the intestinal expression of TLRs and cytokines precedes histological injury in the experimental NEC.

Previously uncharacterized Salmonella enterica genes required for swarming play a role in seedling colonization

Incidences of bacterial foodborne illness caused by ingestion of fresh produce are rising. Instead of this being due to incidental contamination, the animal pathogen Salmonella enterica utilizes specific molecular mechanisms to attach to and colonize plants. This work characterizes two S. enterica genes of unknown function: a putative periplasmic protein, STM0278, and a putative protein with a hydrolase in the C-terminus, STM0650. STM0278 and STM0650 are important for seedling colonization but appear to have different roles during the process of colonization. Mutants of either STM0278 or STM0650 showed reduced colonization of alfalfa seedlings at 24 h, and the STM0278 mutant also showed reduced colonization at 48 h. Both genes were expressed in planta at 4 h following inoculation of 3-day-old seedlings and at 72 h after seed inoculation. This suggests that the role of STM0650 in seedling colonization is less important later in the process or is duplicated by other mechanisms. Mutants of STM0278 and STM0650 were defective in swarming. The STM0278 mutant failed to swarm in 24 h, while swarming of the STM0650 mutant was delayed. Addition of surfactant restored swarming of the STM0278 mutant, suggesting that STM0278 is involved in surfactant or osmotic agent production or deployment. Alfalfa seed exudates as the sole nutrient source were capable of perpetuating S. enterica swarming. Sequence analysis revealed sequences homologous to STM0278 and STM0650 in plant-associated bacteria, but none in Escherichia coli. Phylogenetic analysis of STM0650 showed similar sequences from diverse classes of plant-associated bacteria. Bacteria that preferentially colonize roots, including S. enterica, may use a similar hydrolase for swarming or biofilm production on plants. Multicellular behaviours by S. enterica appear central to plant colonization. S. enterica genes involved in plant colonization and survival outside of a host are most likely among the 'function unknown' genes of this bacterium.