Older leaves of lettuce (Lactuca spp.) support higher levels of Salmonella enterica ser. Senftenberg attachment and show greater variation between plant accessions than do younger leaves

Rain splash-mediated dispersal of Escherichia coli from fecal deposits to field-grown lettuce in the mid- and south Atlantic U.S. regions is affected by mulch type

Introduction

Wildlife feces can contaminate vegetables when enteric bacteria are released by rain and splashed onto crops. Regulations require growers to identify and not harvest produce that is likely contaminated, but U.S. federal standards do not define dimensions for no-harvest zones. Moreover, mulching, used to retain soil moisture and maximize crop yield may impact rain-mediated bacterial dispersal from feces.

Methods

To assess Escherichia coli dissemination from a fecal point source to lettuce grown on various mulches, lettuce cv. 'Magenta' was transplanted into raised beds with plastic, biodegradable plastic, straw, or left uncovered at field sites in Maryland and Georgia. Eleven days post-transplant, 10 g of rabbit manure spiked with ~8 log CFU g-1 E. coli were deposited in each bed. One day following natural or simulated rain events, lettuce was sampled along 1.5 m transects on either side of fecal deposits. Lettuce-associated E. coli was semi-quantified with an MPN assay and dependence on fecal age (stale or fresh), lettuce age (baby leaf or mature head), distance from point source, mulch and post-rain days were statistically evaluated.

Results

Distance (p<0.001), fecal age (p<0.001) and mulch (p<0.01) were factors for E. coli transfer from point source to lettuce. The highest and lowest E. coli estimates were measured from lettuce grown on biodegradable plastic and straw, respectively, with a 2-log MPN difference (p<0.001). Mulch and distance were also significant factors in E. coli recovery 3 days post-rain (both p<0.001), where plastic mulches differed from bare ground and straw (p<0.01). For all treatments, fewer E. coli were retrieved from lettuce at 0.3 m, 3 days post-rain compared to 1 day (p<0.001). Fitting the data to a Weibull Model predicated that a 7-log reduction in E. coli from fecal levels would be achieved at 1.2-1.4 m from the point source on plastic mulches, 0.75 m on bare soil (p<0.05) and 0.43 m on straw (p<0.01).

Discussion

Straw and bare ground limited rain-mediated E. coli dispersal from feces to lettuce compared to plastic mulches. Fecal age was negatively associated with E. coli dispersal. These findings can inform harvesting recommendations for measures related to animal intrusion in vegetable production areas.

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.

Phagotrophic protists preserve antibiotic-resistant opportunistic human pathogens in the vegetable phyllosphere

Food safety of leafy greens is an emerging public health issue as they can harbor opportunistic human pathogens (OHPs) and expose OHPs to consumers. Protists are an integral part of phyllosphere microbial ecosystems. However, our understanding of protist-pathogen associations in the phyllosphere and their consequences on public health remains poor. Here, we examined phyllosphere protists, human pathogen marker genes (HPMGs), and protist endosymbionts from four species of leafy greens from major supermarkets in Xiamen, China. Our results showed that Staphylococcus aureus and Klebsiella pneumoniae were the dominant human pathogens in the vegetable phyllosphere. The distribution of HPMGs and protistan communities differed between vegetable species, of which Chinese chive possessed the most diverse protists and highest abundance of HPMGs. HPMGs abundance positively correlated with the diversity and relative abundance of phagotrophic protists. Whole genome sequencing further uncovered that most isolated phyllosphere protists harbored multiple OHPs which carried antibiotic resistance genes, virulence factors, and metal resistance genes and had the potential to HGT. Colpoda were identified as key phagotrophic protists which positively linked to OHPs and carried diverse resistance and virulence potential endosymbiont OHPs including Pseudomonas nitroreducens, Achromobacter xylosoxidans, and Stenotrophomonas maltophilia. We highlight that phyllosphere protists contribute to the transmission of resistant OHPs through internalization and thus pose risks to the food safety of leafy greens and human health. Our study provides insights into the protist-OHP interactions in the phyllosphere, which will help in food safety surveillance and human health.

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

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

Natural variation and drought-induced differences in metabolite profiles of red oak-leaf and Romaine lettuce play a role in modulating the interaction with Salmonella enterica

Nutrients on produce surfaces are vital for successful enteric pathogen colonisation. In this study, we investigated natural variation in metabolite profiles of Romaine 'Parris Island Cos' and red oak-leaf lettuce 'Mascara' under regular and restricted watering conditions. We also investigated the impact of plant drought stress on the Salmonella - lettuce association. Salmonella Newport and Typhimurium were able to persist at higher levels on regularly watered Romaine than red oak-leaf lettuce. Drought treatment to lettuce impaired epiphytic Salmonella association, with S. Newport and Typhimurium being differentially affected. A higher log reduction of both serotypes was measured on drought-subjected red oak-leaf lettuce plants than controls, but S. Typhimurium was unaffected on water deficit-treated Romaine lettuce (p < 0.05). To assess Salmonella interaction with leaf surface metabolites, leaf washes collected from both cultivars were inoculated and found to be able to support S. Newport growth, with higher levels of Salmonella retrieved from Romaine washes (p < 0.05). The lag phase of S. Newport in washes from water restricted red oak-leaf lettuce was prolonged in relation to regularly-watered controls (p < 0.05). Untargeted plant metabolite profiling using electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) revealed natural variation between Romaine and red oak-leaf lettuce profiles for leaf tissue and leaf washes. Metabolite profile shifts were detected in both lettuce types in response to drought stress, but more unique peaks were detected in red oak-leaf than Romaine lettuce after drought treatment. Variation between the two cultivars was in part attributed to naturally higher levels of flavonoids and anthocyanins in red oak-leaf lettuce compared to Romaine. Moreover, red oak-leaf, but not Romaine lettuce, responded to drought by inducing the accumulation of proline, phenolics, flavonoids and anthocyanins. Drought stress, therefore, enhanced the functional food properties of red oak-leaf lettuce. Salmonella growth dynamics in lettuce leaf washes suggested that natural variation and drought-induced changes in metabolite profiles in lettuce could partly explain the differential susceptibility of various lettuce types to Salmonella, although the primary or secondary metabolites mediating this effect remain unknown. Regulated mild water stress should be investigated as an approach to lower Salmonella contamination risk in suitable lettuce cultivars, while simultaneously boosting the health beneficial quality of lettuce.

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

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

Lysinibacillus macroides-mediated control of cellulose-producing morphotype of Salmonella

BACKGROUND

Soil-dwelling human pathogens like Salmonella are transmitted by fresh produce such as tomato, spinach, onion and cabbage. With >2600 serovars, it is difficult to classify the good plant colonizers from the non-colonizers. Generally, soil microbiota are classified as autochthonous or zymogenous organisms, based on their ability to survive in soil. However, such information for soil-dwelling human pathogens is not available Thus there is a need to classify these organisms for designing a strategy to prevent their outbreak. Moreover, soil harbours a plethora of microbes, which can be screened for competitive organisms to control such human pathogens.

RESULTS

In this study, we examined whether the morphotype based on the attachment factors (e.g., cellulose and curli fimbri) of Salmonella was important for its colonization of roots. Secondly, we tracked the location of the bacteria in the plant cell. Interestingly, most of the epidermal cells occupied by Salmonella showed propidium iodide-positive nuclei. As an extension of the study, a screening of competitive rhizospheric bacteria was performed. One isolate, identified as Lysinibacillus macroides, was able to inhibit the biofilm of Salmonella and subsequently reduced its colonization on roots.

CONCLUSION

Based on this study, we classified the Rdar (red, dry and rough) morphotypes as good plant colonists. The ability to colonize and subsequent kill the live plant cell throws light on the zymogenous life cycle of soil-dwelling Salmonella. Additionally, Lysinibacillus macroides served as a biocontrol agent by reducing the burden of Salmonella in various vegetables. Such organisms can further be explored to prevent contamination of the food chain. © 2022 Society of Chemical Industry.

Contamination of microalgae by Salmonella enterica and Escherichia coli is influenced by selection breeding in chicory (Cichorium intybus L.)

Objectives

The aim of this study was to assess whether selection breeding in chicory (Cichorium intybus L.) led changes in the susceptibility to Salmonella enterica and Escherichia coli contamination and whether the anatomical traits of the leaves are involved in the possible changes.

Materials and Methods

Five chicory genotypes subjected to different intensities of selection were compared at the microgreen stage. Bacterial retention was evaluated after leaf incubation for 1.5 h on the surface of the bacterial suspension, followed by rinsing, grinding, plating on selective media, and CFU counting. The density of stomata and trichomes, total stomatal length and width, stomatal pit width, surface roughness and sharpness were evaluated.

Results

The intensively selected genotype (Witloof) was significantly more prone to contamination ((2.9±0.3) lg CFU/cm 2) as the average of the two bacteril types than the wild accession (Wild) ((2.3±0.4) lg CFU/cm 2) and the moderately selected genotypes (two leaf chicories, Catalogna type, and root chicory ‘Magdeburg’) (on average, (1.9±0.3) lg CFU/cm 2). Witloof microgreens also showed larger stomata (on average + 34% for stoma width and + 44% for pit width), which could justify, at least in part, the higher susceptibility to enterobacteria contamination. In fact, when contamination was performed in the dark (closed stomata), the bacterial retention in Witloof was significantly reduced in comparison with the opened stomata (-44%) and in Wild (-26%). Differences in retention between Witloof and Wild were still observed after UV treatment. The hierarchical clustering performed by grouping the leaf anatomical features was consistent with the chicory genetic groups.

Conclusions

Our results suggest that the domestication process can affect the safety of produce and that the micromorphological traits of the leaves may be involved.

A Double-Edged Sword of Surfactant Effect on Hydrophobic Surface Broccoli Leaf as a Model Plant: Promotion of Pathogenic Microbial Contamination and Improvement to Disinfection Efficiency of Ozonated Water

Pathogenic microbial contamination is significantly influenced by the crop surface properties and surfactant use, which are crucial factors for the postharvest washing process. However, there is little information on the interaction between surfactant and food pathogens on food crops. Thus, this study (1) investigated whether the attachment of Salmonella increases as pesticides denature epicuticular wax crystals and (2) tested if the antibacterial effect of ozonated water can be improved on waxy produce surfaces by adding surfactant to ozonated water. As a result, significantly lower levels of Salmonella Typhimurium attached to waxy leaf surfaces than they did to glossy and pesticide-treated waxy leaf surfaces (3.28 as opposed to 4.10 and 4.32 Log colony forming units (CFU)/cm2, respectively), suggesting that the pesticide containing a surfactant application increased the attachment of S. Typhiumurium on waxy leaf surfaces. There was no significant washing effect on waxy leaf surfaces washed with ozonated water. On the other hand, S. Typhimurium were not detected on waxy leaf surfaces after washing with surfactant-added ozonated water.

Leaf Surface Topography Contributes to the Ability of Escherichia coli on Leafy Greens to Resist Removal by Washing, Escape Disinfection With Chlorine, and Disperse Through Splash

The attachment of foodborne pathogens to leaf surfaces is a complex process that involves multiple physical, chemical, and biological factors. Here, we report the results from a study designed to specifically determine the contribution of spinach leaf surface topography as it relates to leaf axis (abaxial and adaxial) and leaf age (15, 45, and 75 days old) to the ability of Escherichia coli to resist removal by surface wash, to avoid inactivation by chlorine, and to disperse through splash impact. We used fresh spinach leaves, as well as so-called "replicasts" of spinach leaf surfaces in the elastomer polydimethylsiloxane to show that leaf vein density correlated positively with the failure to recover E. coli from surfaces, not only using a simple water wash and rinse, but also a more stringent wash protocol involving a detergent. Such failure was more pronounced when E. coli was surface-incubated at 24°C compared to 4°C, and in the presence, rather than absence, of nutrients. Leaf venation also contributed to the ability of E. coli to survive a 50 ppm available chlorine wash and to laterally disperse by splash impact. Our findings suggest that the topographical properties of the leafy green surface, which vary by leaf age and axis, may need to be taken into consideration when developing prevention or intervention strategies to enhance the microbial safety of leafy greens.

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

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

Abundance of human pathogen genes in the phyllosphere of four landscape plants

The surface of leaf, also known as phyllosphere, harbors diverse microbial communities which include both beneficial microorganisms promoting plants growth and harmful microorganisms, such as plant pathogens and human pathogens. Several studies have investigated the interaction between plants and human pathogens, while few works have focused on the quantitative analysis of pathogenic bacteria. On the basis of real-time polymerase chain reaction (qPCR), this study aimed to evaluate the abundance of following genes: the nuc and pvl of Staphylococcus aureus, the lytA and psaA of Streptococcus pneumoniae, and the ttr and invA of Salmonella enterica in the phyllosphere of four landscape plants (Nandina domestica, Rhododendron pulchrum, Photinia serrulata, and Cinnamomum camphora) growing in two habitats. Our results indicated that the relative abundance of pathogenic genes in the phyllosphere ranged from 10^-9 to 10^-6. The specific genes of S. aureus, S. pneumoniae and S. enterica in landscape plants were pvl, lytA and ttr, respectively. The two pathogenic genes of S. pneumoniae and the 16S rRNA gene were mainly affected by habitats, host species, and habitats-species interaction. Moreover, for the abundance of lytA and 16S rRNA, results showed that plants present in roadside with traffic pollution were relatively higher than that of campus with less pollution. The N. domestica and C. camphora were recommended for planting along the roadsides due to lower abundance of pathogenic genes. However, we have observed no significant difference in the abundance of pathogenic genes among four plants in the campus. Thereby, this study provided a valuable reference for selecting landscape plants in view of human health.

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

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

Breeding Crops for Enhanced Food Safety

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

Salmonella Establishment in Agricultural Soil and Colonization of Crop Plants Depend on Soil Type and Plant Species

Human pathogenic bacteria, such as Salmonella enterica, are able to colonize crop plants. So far, not much is known about biotic and abiotic factors influencing this colonization in field soil. This understanding, however, is imperative for the provision of safe fresh produce to the consumer. In this study, we investigated the effects of soil type, organic fertilization, plant species and the way of Salmonella entry into the plant production system, on the survival of S. enterica in soil as well as the colonization of plants. The selected S. enterica serovar Typhimurium strain 14028s, S. Typhimurium strain LT2 and S. Senftenberg were able to persist in soil for several weeks. Salmonella's persistence in soil was prolonged in loamy, if compared to sandy soil, and when applied together with organic fertilizer. The leaves of lettuce and corn salad were colonized by S. enterica providing evidence for internalization from the soil via the root. Colonization rates were affected by soil type, plant species and S. enterica strain. Overall, S. enterica was detected in leaves of 0.5-0.9% of the plants, while lettuce was more frequently colonized than corn salad. Plants grown in sandy soil were more often colonized than plants grown in loamy soil. After spray inoculation, S. enterica could be detected on and in leaves for several weeks by cultivation-depending methods, confirmed by confocal microscopy using GFP-labeled S. Typhimurium 14028s. On the one hand, transcriptome data from S. Typhimurium 14028s assessed in response to lettuce medium or lettuce root exudates showed an upregulation of genes associated with biofilm formation and virulence. On the other hand, lettuce inoculated with S. Typhimurium 14028s showed a strong upregulation of genes associated with plant immune response and genes related to stress response. In summary, these results showed that organic fertilizers can increase the persistence of Salmonella in soil and that soil type and plant species play a crucial role in the interactions between human pathogens and crop plants. This understanding is therefore a starting point for new strategies to provide safe food for the consumer.

Survival and internalization of Salmonella and Escherichia coli O157:H7 sprayed onto different cabbage cultivars during cultivation in growth chambers

BACKGROUND

Cabbage may become contaminated with enteric pathogens during cultivation. Using multiple cabbage cultivars at two maturity stages (small plants or plants with small heads) in growth chamber studies, the fate (internalization or surface survival) of Salmonella and Escherichia coli O157:H7 (0157) were examined in conjunction with any potential relationships to the plant's antimicrobial content.

RESULTS

Internalized Salmonella was detected in cabbage within 24 h with prevalence ranging from 62% (16 of 26) for the 'Super Red 80' cultivar to 92% (24 of 26) for the 'Red Dynasty' cultivar. Surface survival of pathogens on small cabbage plants over nine days was significantly affected by cultivar with both pathogens surviving the most on the 'Farao' cultivar and Salmonella and O157 surviving the least on the 'Super Red 80' and 'Capture' cultivars, respectively (P < 0.05). Survival of O157 was slightly higher on cabbage heads for O157 than small plants suggesting that the maturity stage may affect this pathogen's fate. An inverse relationship existed between antimicrobial levels and the pathogen's surface survival on cabbage heads (P < 0.05).

CONCLUSIONS

The fate of pathogens varied with the cabbage cultivar in growth chamber studies highlighting the potential to explore cultivar in field studies to reduce the risk of microbiological contamination in this crop. © 2019 Society of Chemical Industry.

Survival of Salmonella enterica and Escherichia coli O157:H7 Sprayed onto the Foliage of Field-Grown Cabbage Plants

To reduce the number of cabbage pathogen outbreaks, it is essential to understand the fate of enteric pathogens that contaminate plants in the field. To assist in that effort, two independent trials were conducted with a red cultivar (cv. Red Dynasty) and a green cultivar (cv. Bravo F1) of field-grown cabbage ( Brassica oleracea var. capitata). In the first trial, plants with small heads were sprayed with an inoculum containing both attenuated Salmonella enterica Typhimurium and Escherichia coli O157:H7 (5.0 log CFU/mL). Initial pathogen levels (ca. 3.9 log CFU per head), determined through plate count enumeration (limit of detection was 1.3 log CFU/g), dropped precipitously such that 2 days later, they could not be detected by enrichment culture in 22 to 35% of the heads. However, subsequent declines were at a slower rate; no differences were observed between red and green cabbage heads ( P > 0.05), and heads were still positive for the pathogens 22 days after being sprayed with the inoculum. As a result, the logistic model revealed that for every 2 days contaminated cabbage heads remained in the field, the probability of finding a positive sample decreased by a factor of 1.1 (95% confidence interval from 1.0 to 1.2, P = 0.0022) and 1.2 (95% confidence interval from 1.0 to 1.4, P ≤ 0.0001) for Salmonella and E. coli O157:H7, respectively. In the second trial occurring 2 weeks later, plants with medium red or green cabbage heads were sprayed with an inoculum at a dose of 3.5 log CFU/mL. A similar decay in prevalence over time occurred for green cabbage as in trial 1; however, pathogen decline in red cabbage was less in trial 2 than in trial 1. The extended persistence of pathogens in cabbage heads exhibited in both trials infers that harvest of contaminated cabbage destined for raw consumption is risky. Additional field studies are necessary to determine whether similar pathogen fates occur in other regions or climates and to clarify the effect of the maturity of red cabbage on pathogen inactivation.

Pre-harvest internalization and surface survival of Salmonella and Escherichia coli O157:H7 sprayed onto different lettuce cultivars under field and growth chamber conditions

Plant genotype has been advocated to have an important role in the fate of enteric pathogens residing in lettuce foliage. This study was therefore undertaken under the premise that different pathogen responses could occur in lettuce cultivars with cultivar selection being one of several hurdles in an overall strategy for controlling foodborne pathogens on field-grown produce. Up to eight lettuce cultivars ('Gabriella', 'Green Star', 'Muir', 'New Red Fire', 'Coastal Star', 'Starfighter', 'Tropicana', and 'Two Star') were examined in these experiments in which the plants were subjected to spray contamination of their foliage with pathogens. In an experiment that addressed internalization of Salmonella, cultivar was determined to be a significant variable (P < 0.05) with 'Gabriella' and 'Muir' being the least and most likely to exhibit internalization of this pathogen, respectively. Furthermore, antimicrobials (total phenols and antioxidant capacity chemicals) could be part of the plant's defenses to resist internalization as there was an inverse relationship between the prevalence of internalization at 1 h and the levels of these antimicrobials (r = -0.75 to -0.80, P = 0.0312 to 0.0165). Internalized cells appeared to be transient residents in that across all cultivars, plants sampled 1 h after being sprayed were 3.5 times more likely to be positive for Salmonella than plants analyzed 24 h after spraying (95% CI from 1.5 to 8.2, P = 0.0035). The fate of surface-resident Salmonella and Escherichia coli O157:H7 was addressed in subsequent growth chamber and field experiments. In the growth chamber study, no effect of cultivar was manifested on the fate of either pathogen when plants were sampled up to 12 days after spray contamination of their foliage. However, in the field study, five days after spraying the plants, Salmonella contamination was significantly affected by cultivar (P < 0.05) and the following order of prevalence of contamination was observed: 'Muir' < 'Gabriella' < 'Green Star' = 'New Red Fire' < 'Coastal Star'. Nine days after spray contamination of plants in the field, no effect of cultivar was exhibited due primarily to the low prevalence of contamination observed for Salmonella (8 of 300 plant samples positive by enrichment culture) and E. coli O157 (4 of 300 plant samples positive by enrichment culture). Given the narrow window of time during which cultivar differences were documented, it is unlikely that cultivar selection could serve as a viable option for reducing the microbiological risk associated with lettuce.

Is There a Relation between the Microscopic Leaf Morphology and the Association of Salmonella and Escherichia coli O157:H7 with Iceberg Lettuce Leaves?

To prevent contamination of fresh produce with enteric pathogens, more insight into mechanisms that may influence the association of these pathogens with fresh produce is needed. In this study, Escherichia coli O157:H7 and Salmonella were chosen as model pathogens, and fresh cut iceberg lettuce was chosen as a model fresh produce type. The morphological structure of iceberg lettuce leaves (stomatal density and length of cell margins per leaf area) was quantified by means of leaf peels and light microscopy of leaves at different stages of development (outer, middle, and inner leaves of the crop) on both leaf sides (abaxial and adxial) and in three leaf regions (top, center, and bottom). The morphology of the top region of the leaves was distinctly different from that of the center and base, with a significantly higher stomatal density (up to five times more stomata), different cell shape, and longer cell margins (two to three times longer). Morphological differences between the same regions of the leaves at different stages of development were smaller or nonsignificant. An attachment assay with two attenuated E. coli O157:H7 strains (84-24h11-GFP and BRMSID 188 GFP) and two Salmonella strains (serovars Thompson and Typhimurium) was performed on different regions of the middle leaves. Our results confirmed earlier reports that these pathogens have a higher affinity for the base of the lettuce leaf than the top. Differences of up to 2.12 log CFU/g were seen ( E. coli O157:H7 86-24h11-GFP). Intermediate attachment occurred in the central region. The higher incidence of preferential bacterial attachment sites such as stomata and cell margins or grooves could not explain the differences observed in the association of the tested pathogens with different regions of iceberg lettuce leaves.