Effects of plant maturity and growth media bacterial inoculum level on the surface contamination and internalization of Escherichia coli O157:H7 in growing spinach leaves

Minimizing Escherichia coli O157:H7 contamination in indoor farming: effects of cultivar type and ultra-violet light quality

BACKGROUND

Bacterial contamination of produce is a concern in indoor farming due to close plant spacing, recycling irrigation, warm temperatures, and high relative humidity during production. Cultivars that inherently resist contamination and photo-sanitization using ultraviolet (UV) radiation during the production phase can reduce bacterial contamination. However, there is limited information to support their use in indoor farming.

RESULTS

Lettuce (Lactuca sativa) cultivars with varying plant architectures grown in a custom-built indoor farm exhibited differences in E. coli O157:H7 survival after inoculation. The survival of E. coli O157:H7 was lowest in the leaf cultivar (open architecture) and highest in the romaine and oakleaf cultivars (compact architecture). Of the different UV wavelengths that were tested (UV-A, UV-A + B, UV-A + C), UV A + C at an intensity of 54.5 μmol m-2 s-1 (with 3.5 μmol m-2 s-1 of UV-C), provided for 15 min every day, was found to be most efficacious in reducing the E. coli O157:H7 survival on romaine lettuce with no negative effects on plant growth and quality.

CONCLUSION

Contamination of E. coli O157:H7 on lettuce plants can be reduced and the food safety levels in indoor farms can be increased by selecting cultivars with an open leaf architecture coupled with photo-sanitization using low and frequent exposure to UV A + C radiation. © 2024 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

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.

Persistence and transfer of Tulane virus in a microgreen cultivation system

Microgreens are niche salad greens which have increased in popularity among consumers in recent years. Due to similarities with sprouts and leafy greens-both attributed to numerous foodborne disease outbreaks-characterization of the food safety risks associated with microgreen production is warranted. The present study aimed to determine the fate and persistence of a human norovirus (HuNoV) surrogate, Tulane virus (TV), within a microgreen production system. Initially, the persistence of TV in two types of microgreen soil-free cultivation matrix (SFCM)-BioStrate® (biostrate) and peat-was determined. On day 0, water containing 7.6 log PFU of TV was applied to SFCM in growing trays, and the trays were maintained under microgreen growth conditions. TV persisted throughout the 10-day observation in biostrate and peat with overall reductions of 3.04 and 1.76 log plaque forming units (PFU) per tray, respectively. Subsequently, the transfer of TV to microgreen edible tissue was determined when planted on contaminated SFCM. Trays containing each type of SFCM were pre-inoculated with 7.6 log PFU of TV and equally divided into two areas. On day 0, sunflower (SF) or pea shoot (PS) seeds were planted on one-half of each tray, while the other half was left unplanted to serve as a control. The microgreens were harvested on day 10, and SFCM samples were collected from planted and unplanted areas of each tray. No TV were detected from the edible portion of either type of microgreen, yet TV were still present in the SFCM. TV concentrations were significantly lower in the root-containing planted area compared with the unplanted area for both biostrate (P = 0.0282) and peat (P = 0.0054). The mean differences of TV concentrations between unplanted and planted areas were 1.22 and 0.51 log PFU/g for biostrate and peat, respectively. In a subsequent investigation, TV transfer from day 7 inoculated SFCM to microgreens edible portion was not detected either. Overall, this study characterized the viral risk in a microgreen production system, which will help to understand the potential food safety risk related to HuNoV and to develop preventive measures.

Impact of population density and stress adaptation on the internalization of Salmonella in leafy greens

Salmonella enterica is capable of entering the interior of leafy greens and establishing in the apoplastic area, a phenomenon known as internalization. The ability of internalized bacteria to evade common disinfection practices poses a well-established risk. Our aim was to study the effect of: i) inoculum size and ii) prior adaptation of Salmonella to sublethal stresses, on the internalization of the pathogen in four leafy vegetables. Spinach, lettuce, arugula and chicory were inoculated, by immersion for 2 min at room temperature with: i) Salmonella Enteritidis at 3.0, 4.0, 5.0, 6.0, 7.0 log CFU/mL and ii) non-adapted or adapted S. Enteritidis to acid (in TSB with 1% glucose, incubated for 24 h at 37 °C), cold (in TSB for 7 days at 4 °C), starvation (0.85% NaCl of pH 6.6, 48 h at 37 °C) or desiccation (1.5 h at 42 °C, 4 days at 21 °C) stress at appx 3.5 log CFU/mL). Inoculated leafy greens were subsequently stored at 5 °C and 20 °C for 2 h and 48 h (n = 2 × 2). Population of internalized Salmonella, after surface decontamination with 1% w/v AgNO3, was assessed on selective media. Even the lowest initial bacterial inoculum was adequate for internalization of Salmonella to occur in leafy vegetables. Non-adapted Salmonella inoculum of 7.0 (maximum) and 3.0 log CFU/mL (lowest inoculation level tested) after short storage (2 h) resulted in 3.7-4.3 and 1.3-1.5 log CFU/g internalized bacterial population, respectively. Colonization (including both attachment and internalization processes), as well as internalization process, were positively correlated to initial inoculum level. These processes reached a different plateau beyond which, no further increase in internalization was observed. Adaptation of the pathogen to mild stresses enhanced internalization (P < 0.05), with desiccation- and acid-adapted Salmonella demonstrating the highest internalization capacity, regardless of the vegetable and storage temperature. These findings could contribute to further elucidation of colonization capacity of Salmonella in leafy vegetables and assist in selecting the proper conditions that contribute to the prevention of fresh produce contamination with Salmonella.

Diversity of bacterial endophyte in Eucalyptus clones and their implications in water stress tolerance

The genus Eucalyptus with over 747 species occurs in wide ecological range and is preferred for bioenergy plantations due to their short rotation, rapid growth and superior wood properties. They are planted in 22 million ha area and India is third largest planter of Eucalyptus. In the present study, the bacterial endophyte community in leaves of six Eucalyptus clones belonging to E. tereticornis and E. camaldulensis was assessed by sequencing the V3-V4 region of the bacterial 16S rRNA gene. The clones were selected based on their response to progressive water stress. A total of 4947 operational taxonomic units (OTUs) were obtained and the dominant phyla were Proteobacteria, Bacteroidetes and Firmicutes. Escherichia coli was enriched in all samples at species level. Comparison of endophyte diversity was conducted between the two species and across the water stress tolerant and susceptible clones. The alpha-diversity analysis revealed that species richness and diversity was high in E. camaldulensis and water stress susceptible clones. LefSe analysis predicted 69 and 54 significantly enriched taxonomic biomarkers between species and stress response groups respectively. A maximum of 49 taxonomic biomarkers were recorded in susceptible group and the significantly enriched species were Bacteroides thetaiotaomicron and Turicibacter sanguinis, while the tolerant group documented 5 biomarkers including oscillibacter sp. The presence of functional biomarkers was also assessed in both the groups. The findings of the present study provides an insight into the diversity of bacterial endophyte in Eucalyptus leaves and to our knowledge this is the first report on documenting the endophyte abundance in water stress responsive Eucalyptus clones.

Salmonella enterica Growth Conditions Influence Lettuce Leaf Internalization

Human pathogens on plants (HPOP) have evolved complex interactions with their plant host. Stomatal internalization is one such mode of interaction, where bacteria are attracted to stomata and penetrate into the substomatal cavity by a process mediated by chemotaxis. Internalization enables HPOP to evade the hostile environment of the leaf surface and find a protected, nutrient-rich niche within the leaf. Numerous studies have documented attachment and entry of the foodborne pathogens, Salmonella enterica and Escherichia coli into stomata. Internalization, however, varies considerably among different pathogens and in different plants, and both bacterial and plant’s factors were reported to influence HPOP attachment and internalization. Here we have studied the effect of laboratory growth conditions, on the internalization of Salmonella enterica serovar Typhimurium (STm) into lettuce leaf. We have further tested the potential involvement of universal stress-proteins in leaf internalization. We found that STm grown in Luria Bertani broth devoid of NaCl (LBNS), or in diluted LB (0.5×LB) internalized lettuce leaf better (62 ± 5% and 59 ± 7%, respectively) compared to bacteria grown in LB (15 ± 7%). Growth under non-aerated conditions also enhanced STm internalization compared to growth under aerated conditions. Growth temperature of 25 and 37°C did not affect STm internalization, however, growth at 42°C, significantly augmented leaf internalization. Since, the tested growth conditions represent moderate stresses, we further investigated the involvement of five universal-stress genes in STm leaf internalization following growth in LBNS medium. Knockout mutations in ydaA, yecG, ybdQ, and uspAB, but not in ynaF, significantly reduced STm internalization compared to the wild-type (wt) strain, without affecting bacterial attachment and motility. Transduction of the mutations back to the parent strain confirmed the linkage between the mutations and the internalization phenotype. These findings support a specific role of the universal-stress genes in leaf internalization. The present study highlights the complexity of bacterial internalization process and may provide partial explanation for the variable, sometimes-contrasting results reported in the literature regarding stomatal internalization by HPOP. Characterization of the regulatory networks that mediate the involvement of usp genes and the tested growth factors in STm internalization should contribute to our understanding of human pathogens-plant interactions.

Evaluation of post-contamination survival and persistence of applied attenuated E. coli O157:H7 and naturally-contaminating E. coli O157:H7 on spinach under field conditions and following postharvest handling

This study determined the variability in population uniformity of an applied mixture of attenuated E. coli O157:H7 (attEcO157) on spinach leaves as impacted by sampling mass and detection technique over spatial and temporal conditions. Opportunistically, the survival and distribution of naturally contaminating pathogenic E. coli O157:H7 (EcO157), in a single packaged lot following commercial postharvest handling and washing, was also evaluated. From the main study outcomes, differences in the applied inoculum dose of 100-fold, resulted in indistinguishable population densities of approximately Log 1.1 CFU g^-1 by 14 days post-inoculation (DPI). Composite leaf samples of 150 g and the inclusion of the spinach petiole resulted in the greatest numerical sensitivity of detection of attEcO157 when compared to 25 and 150 g samples without petioles (P < 0.05). Differences in population density and protected-site survival and potential leaf internalization were observed between growing seasons and locations in California (P < 0.05). A Double Weibull model best described and identified two distinct populations with different inactivation rates of the inoculated attEcO157. Linear die-off rates varied between 0.14 and 0.29 Log/Day irrespective of location. Detection of EcO157- stx1-negative and stx2-positive, resulting from a natural contamination event, was observed in 11 of 26 quarantined commercial units of washed spinach by applying the 150 g sample mass protocol. The capacity to detect EcO157 varied between commercial test kits and non-commercial qPCR. Our findings suggest the need for modifications to routine pathogen sampling protocols employed for lot acceptance of spinach and other leafy greens.

[Persistence, internalization and translocation of Escherichia coli O157:H7, O157:H16 and O105ab in plants and tomato fruits (Solanum lycopersicum L.)]

The presence of pathogenic bacteria, such as Escherichia coli affects the quality and safety of vegetables that are consumed fresh and is associated with serious health problems. The objective of this study was to determine if three different strains of E. coli can penetrate and remain in plants and tomato fruits. A completely randomized experimental design was followed for which a tomato crop ("Cid" variety) was established under greenhouse conditions and three treatments were evaluated, T1 (E. coli O157:H7), T2 (E. coli from tomato cultivation [EcT] O157:H16), T3 (E. coli from spinach cultivation [EcH] O105ab) and a T4 control, with 100 plants each and four forms of inoculation: in the substrate, steam, petiole and the peduncle. Samples were carried out in vegetative stage, flowering, fruiting and physiological maturity to quantify in petri dish CFU/g and know if the bacteria managed to move around and recover in root, stem, flower and fruit. The phylogenetic groups that corresponded to the bacteria recovered were confirmed by biochemical tests, serotyping and PCR. At 120 days the recovery of bacteria in the plant was 23% (E. coli O157:H7), 28% (EcT O157:H16) and 55% (EcH O105ab) whit inoculation to the substrate while the inoculation by puncture the recovery was (in the same order) of 5%, 3%, and 4% at 30 days; 37%, 35% and 30% at 90 days; and 42%, 39% and 13% at 65 days. The strains submit the ability to enter the tomato plant and to stay in it and transported to the fruit, without producing that indicate their presence.

Internalization and dissemination of human norovirus and Tulane virus in fresh produce is plant dependent

Human norovirus (NoV) is a leading cause of fresh produce associated outbreaks. Previous research indicates that the roots of growing leafy greens and berries internalize human NoV. However the effect of plant type and inoculum level on internalization rates has not been directly compared. In this study we compared the internalization and dissemination rates of human NoV and its surrogate, Tulane virus (TV) in green onion, radishes, and Romaine lettuce. We also evaluated the effect inoculum level and plant growth matrix on the rate of viral internalization. In the hydroponic growth system, we detected internalization and dissemination of human NoV RNA in green onions. In hydroponically growing green onions inoculated with high titer TV, we found higher rates of internalization and dissemination compared to green onions inoculated with low titer TV. In soil growth systems, no infectious TV was detected in either green onion or radishes. However, in Romaine lettuce plants grown in soil approximately 4 log10 PFU/g was recovered from all tissues on day 14 p.i. Overall, we found that the type of plant, growth matrix, and the inoculum level influences the internalization and dissemination of human NoV and TV.

Soil Conditions That Can Alter Natural Suppression of Escherichia coli O157:H7 in Ohio Specialty Crop Soils

Food-borne pathogen persistence in soil fundamentally affects the production of safe vegetables and small fruits. Interventions that reduce pathogen survival in soil would have positive impacts on food safety by minimizing preharvest contamination entering the food chain. Laboratory-controlled studies determined the effects of soil pH, moisture content, and soil organic matter (SOM) on the survivability of this pathogen through the creation of single-parameter gradients. Longitudinal field-based studies were conducted in Ohio to quantify the extent to which field soils suppressed Escherichia coli O157:H7 survival. In all experiments, heat-sensitive microorganisms were responsible for the suppression of E. coli O157 in soil regardless of the chemical composition of the soil. In laboratory-based studies, soil pH and moisture content were primary drivers of E. coli O157 survival, with increases in pH after 48 h (P = 0.02) and decreases in moisture content after 48 h (P = 0.007) significantly increasing the log reduction of E. coli O157 numbers. In field-based experiments, E. coli O157 counts from both heated and unheated samples were sensitive to both season (P = 0.004 for heated samples and P = 0.001 for unheated samples) and region (P = 0.002 for heated samples and P = 0.001 for unheated samples). SOM was observed to be a more significant driver of pathogen suppression than the other two factors after 48 h at both planting and harvest (P = 0.002 at planting and P = 0.058 at harvest). This research reinforces the need for both laboratory-controlled experiments and longitudinal field-based experiments to unravel the complex relationships controlling the survival of introduced organisms in soil.

Evidence of the internalization of animal caliciviruses via the roots of growing strawberry plants and dissemination to the fruit

Human norovirus (NoV) is the leading cause of foodborne disease in the United States, and epidemiological studies have shown that fresh produce is one of the major vehicles for the transmission of human NoV. However, the mechanisms of norovirus contamination and persistence in fresh produce are poorly understood. The objective of this study is to determine whether human NoV surrogates, murine norovirus (MNV-1) and Tulane virus (TV), can attach and become internalized and disseminated in strawberries grown in soil. The soil of growing strawberry plants was inoculated with MNV-1 and TV at a level of 10(8) PFU/plant. Leaves and berries were harvested over a 14-day period, and the viral titer was determined by plaque assay. Over the course of the study, 31.6% of the strawberries contained internalized MNV-1, with an average titer of 0.81 ± 0.33 log10 PFU/g. In comparison, 37.5% of strawberries were positive for infectious TV, with an average titer of 1.83 ± 0.22 log10 PFU/g. A higher percentage (78.7%) of strawberries were positive for TV RNA, with an average titer of 3.15 ± 0.51 log10 RNA copies/g as determined by real-time reverse transcriptase quantitative PCR (RT-qPCR). In contrast, no or little virus internalization and dissemination were detected when TV was inoculated into bell peppers grown in soil. Collectively, these data demonstrate (i) virally contaminated soils can lead to the internalization of virus via plant roots and subsequent dissemination to the leaf and fruit portions of growing strawberry plants and (ii) the magnitude of internalization is dependent on the type of virus and plant.

Effect of proximity to a cattle feedlot on Escherichia coli O157:H7 contamination of leafy greens and evaluation of the potential for airborne transmission

The impact of proximity to a beef cattle feedlot on Escherichia coli O157:H7 contamination of leafy greens was examined. In each of 2 years, leafy greens were planted in nine plots located 60, 120, and 180 m from a cattle feedlot (3 plots at each distance). Leafy greens (270) and feedlot manure samples (100) were collected six different times from June to September in each year. Both E. coli O157:H7 and total E. coli bacteria were recovered from leafy greens at all plot distances. E. coli O157:H7 was recovered from 3.5% of leafy green samples per plot at 60 m, which was higher (P < 0.05) than the 1.8% of positive samples per plot at 180 m, indicating a decrease in contamination as distance from the feedlot was increased. Although E. coli O157:H7 was not recovered from air samples at any distance, total E. coli was recovered from air samples at the feedlot edge and all plot distances, indicating that airborne transport of the pathogen can occur. Results suggest that risk for airborne transport of E. coli O157:H7 from cattle production is increased when cattle pen surfaces are very dry and when this situation is combined with cattle management or cattle behaviors that generate airborne dust. Current leafy green field distance guidelines of 120 m (400 feet) may not be adequate to limit the transmission of E. coli O157:H7 to produce crops planted near concentrated animal feeding operations. Additional research is needed to determine safe set-back distances between cattle feedlots and crop production that will reduce fresh produce contamination.

The growing season, but not the farming system, is a food safety risk determinant for leafy greens in the mid-Atlantic region of the United States

Small- and medium-size farms in the mid-Atlantic region of the United States use varied agricultural practices to produce leafy greens during spring and fall, but the impact of preharvest practices on food safety risk remains unclear. To assess farm-level risk factors, bacterial indicators, Salmonella enterica, and Shiga toxin-producing Escherichia coli (STEC) from 32 organic and conventional farms were analyzed. A total of 577 leafy greens, irrigation water, compost, field soil, and pond sediment samples were collected. Salmonella was recovered from 2.2% of leafy greens (n = 369) and 7.7% of sediment (n = 13) samples. There was an association between Salmonella recovery and growing season (fall versus spring) (P = 0.006) but not farming system (organic or conventional) (P = 0.920) or region (P = 0.991). No STEC was isolated. In all, 10% of samples were positive for E. coli: 6% of leafy greens, 18% of irrigation water, 10% of soil, 38% of sediment, and 27% of compost samples. Farming system was not a significant factor for levels of E. coli or aerobic mesophiles on leafy greens but was a significant factor for total coliforms (TC) (P < 0.001), with higher counts from organic farm samples. Growing season was a factor for aerobic mesophiles on leafy greens (P = 0.004), with higher levels in fall than in spring. Water source was a factor for all indicator bacteria (P < 0.001), and end-of-line groundwater had marginally higher TC counts than source samples (P = 0.059). Overall, the data suggest that seasonal events, weather conditions, and proximity of compost piles might be important factors contributing to microbial contamination on farms growing leafy greens.

Biotic and abiotic variables affecting internalization and fate of Escherichia coli O157:H7 isolates in leafy green roots

Preharvest internalization of Escherichia coli O157:H7 into the roots of leafy greens is a food safety risk because the pathogen may be systemically transported to edible portions of the plant. In this study, both abiotic (degree of soil moisture) and biotic (E. coli O157:H7 exposure, presence of Shiga toxin genes, and type of leafy green) factors were examined to determine their potential effects on pathogen internalization into roots of leafy greens. Using field soil that should have an active indigenous microbial community, internalized populations in lettuce roots were 0.8 to 1.6 log CFU/g after exposure to soil containing E. coli O157:H7 at 5.6 to 6.1 log CFU/g. Internalization of E. coli O157:H7 into leafy green plant roots was higher when E. coli O157:H7 populations in soil were increased to 7 or 8 log CFU/g or when the soil was saturated with water. No differences were noted in the extent to which internalization of E. coli O157:H7 occurred in spinach, lettuce, or parsley roots; however, in saturated soil, maximum levels in parsley occurred later than did those in spinach or lettuce. Translocation of E. coli O157:H7 from roots to leaves was rare; therefore, decreases observed in root populations over time were likely the result of inactivation within the plant tissue. Shiga toxin-negative (nontoxigenic) E. coli O157:H7 isolates were more stable than were virulent isolates in soil, but the degree of internalization of E. coli O157:H7 into roots did not differ between isolate type. Therefore, these nontoxigenic isolates could be used as surrogates for virulent isolates in field trials involving internalization.

Impact of phytopathogen infection and extreme weather stress on internalization of Salmonella Typhimurium in lettuce

Internalization of human pathogens, common in many types of fresh produce, is a threat to human health since the internalized pathogens cannot be fully inactivated/removed by washing with water or sanitizers. Given that pathogen internalization can be affected by many environmental factors, this study was conducted to investigate the influence of two types of plant stress on the internalization of Salmonella Typhimurium in iceberg lettuce during pre-harvest. The stresses were: abiotic (water stress induced by extreme weather events) and biotic (phytopathogen infection by lettuce mosaic virus [LMV]). Lettuce with and without LMV infection were purposefully contaminated with green fluorescence protein-labeled S. Typhimurium on the leaf surfaces. Lettuce was also subjected to water stress conditions (drought and storm) which were simulated by irrigating with different amounts of water. The internalized S. Typhimurium in the different parts of the lettuce were quantified by plate count and real-time quantitative PCR and confirmed with a laser scanning confocal microscope. Salmonella internalization occurred under the conditions outlined above; however internalization levels were not significantly affected by water stress alone. In contrast, the extent of culturable S. Typhimurium internalized in the leafy part of the lettuce decreased when infected with LMV under water stress conditions and contaminated with high levels of S. Typhimurium. On the other hand, LMV-infected lettuce showed a significant increase in the levels of culturable bacteria in the roots. In conclusion, internalization was observed under all experimental conditions when the lettuce surface was contaminated with S. Typhimurium. However, the extent of internalization was only affected by water stress when lettuce was infected with LMV.

Differences in biofilm formation of produce and poultry Salmonella enterica isolates and their persistence on spinach plants

Spinach plants were irrigated biweekly with water containing 2.1 log CFU Salmonella/100 ml water (the maximum Escherichia coli MPN recommended by the Leafy Greens Marketing Agreement; LGMA), or 4.1 CFU Salmonella/100 ml water to determine Salmonella persistence on spinach leaves. Green Fluorescent protein expressing Salmonella were undetectable by most-probable number (MPN) at 24 h and 7 days following each irrigation event. This study indicates that Salmonella are unlikely to persist on spinach leaves when irrigation water is contaminated at a level below the LGMA standards. In a parallel study, persistence of Salmonella isolated from poultry or produce was compared following biweekly irrigation of spinach plants with water containing 6 log CFU Salmonella/100 ml. Produce Salmonella isolates formed greater biofilms on polystyrene, polycarbonate and stainless steel surfaces and persisted at significantly higher numbers on spinach leaves than those Salmonella from poultry origin during 35 days study. Poultry Salmonella isolates were undetectable (<1 log CFU/g) on spinach plants 7 days following each irrigation event when assayed by direct plating. This study indicates that Salmonella persistence on spinach leaves is affected by the source of contamination and the biofilm forming ability of the strain.

Comparison between ImmunoCard STAT!(®) and real-time PCR as screening tools for both O157:H7 and non-O157 Shiga toxin-producing Escherichia coli in Southern Alberta, Canada

An increasing number of non-O157 Shiga toxin-producing Escherichia coli (STEC) infections and outbreaks have been reported. In this study, we evaluated the performance of ImmunoCard STAT!(®) (Meridian Bioscience, Inc., Cincinnati, OH, USA) as a method to screen stool specimens for STEC (O157 and non-O157). An in-house real-time PCR method was used as the "gold standard". We also evaluated the prevalence and clinical characteristics of STEC infections in the Alberta South West Zone. From July to November 2011, 819 stool specimens submitted for routine stool culture were tested. With our in-house real-time PCR, 7 O157:H7 and 10 non-O157 STEC isolates were identified for a total of 17 STECs. In comparison, ImmunoCard STAT!(®) identified a total of 6, resulting in a sensitivity and specificity of 35% and 99%, respectively (P < 0.05). Because of the low sensitivity, ImmunoCard STAT!(®) cannot be recommended as a routine screening test for STEC from enriched stool specimens. The rate of STEC positivity as detected by PCR was 2.08%, of which 0.86% was O157:H7 and 1.22% non-O157 STEC. Five of the 7 cases of STEC O157 infection experienced bloody diarrhea, and 1 developed hemolytic uremic syndrome.

Postharvest handling conditions affect internalization of Salmonella in baby spinach during washing

Internalization of foodborne pathogens in fruits and vegetables is an increasing safety concern. The aim of this research was to assess the potential for internalization of an enteric pathogen (Salmonella enterica serotype Typhimurium) in a leafy vegetable (baby spinach) during washing as influenced by three postharvest handling conditions: (i) illumination, (ii) negative temperature differential, and (iii) relative humidity (RH). To compare these potential postharvest handling conditions, leaves were exposed to different levels of illumination (0, 1,000, and 2,000 lx), temperature differential (5, 11, 14, 20, and 26°C), and RH (99, 85, and 74%) for a short time before or during washing. Washing of baby spinach was carried out in water containing green fluorescent protein-tagged Salmonella Typhimurium (6.5 log CFU/ml) at 5°C for 2 min, followed by surface disinfection with chlorine (10,000 μg/ml) for 1 min, two rinses in water for 10 s, and spin drying for 15 s. Internalization was assessed by enumerating the pathogen on Salmonella-Shigella agar and by confocal laser scanning microscopy. Illumination of spinach leaves before and during washing and a negative temperature differential during washing did not significantly (P > 0.05) increase the number of internalized bacteria. However, exposure of leaves to low-RH conditions before washing, which reduced the tissue water content, decreased internalization of Salmonella compared with internalization in baby spinach exposed to high RH (P ≤ 0.05). Green fluorescent protein-tagged Salmonella Typhimurium was visualized by confocal laser scanning microscopy at a depth of up to 30 m m beneath the surface of spinach leaves after exposure to a high inoculum level (8 log CFU/ml) for an extended time (2 h). Results show that internalization of Salmonella into baby spinach leaves can occur but can be minimized under specific postharvest handling conditions such as low RH.

Generic Escherichia coli contamination of spinach at the preharvest stage: effects of farm management and environmental factors

The objective of this study was to determine the effects of farm management and environmental factors on preharvest spinach contamination with generic Escherichia coli as an indicator of fecal contamination. A repeated cross-sectional study was conducted by visiting spinach farms up to four times per growing season over a period of 2 years (2010 to 2011). Spinach samples (n = 955) were collected from 12 spinach farms in Colorado and Texas as representative states of the Western and Southwestern United States, respectively. During each farm visit, farmers were surveyed about farm-related management and environmental factors using a questionnaire. Associations between the prevalence of generic E. coli in spinach and farm-related factors were assessed by using a multivariable logistic regression model including random effects for farm and farm visit. Overall, 6.6% of spinach samples were positive for generic E. coli. Significant risk factors for spinach contamination with generic E. coli were the proximity (within 10 miles) of a poultry farm, the use of pond water for irrigation, a >66-day period since the planting of spinach, farming on fields previously used for grazing, the production of hay before spinach planting, and the farm location in the Southwestern United States. Contamination with generic E. coli was significantly reduced with an irrigation lapse time of >5 days as well as by several factors related to field workers, including the use of portable toilets, training to use portable toilets, and the use of hand-washing stations. To our knowledge, this is the first report of an association between field workers' personal hygiene and produce contamination with generic E. coli at the preharvest level. Collectively, our findings support that practice of good personal hygiene and other good farm management practices may reduce produce contamination with generic E. coli at the preharvest level.

Localization of viable Salmonella typhimurium internalized through the surface of green onion during preharvest

Internalization of pathogens poses a tremendous health risk in the consumption of raw fresh produce, because conventional washing cannot remove pathogens effectively after internalization occurs. We investigated (i) the pattern of Salmonella internalization in different parts of green onions when it was contaminated on their surfaces, and (ii) whether environmental factors (extreme weather) affect the extent of Salmonell a internalization. Green onions were surface contaminated with three different levels of Salmonella Typhimurium (1, 3, and 5 log CFU per green onion). Each contamination group was irrigated with three different water volumes to mimic water stress and to determine if Salmonella Typhimurium internalization was localized in different parts of the plant. The plants were collected 2 days after contamination, and surface bacteria were inactivated with ethanol and silver nitrate. The plants were then cut into two parts, upper and lower. The internalized Salmonella Typhimurium in each part was visualized and confirmed with a laser scanning confocal microscope and was quantified with the plate count method and real-time quantitative PCR (qPCR). The results indicate that Salmonella Typhimurium can be taken up through the plant surface and transported from the upper to the lower part of the plant. The level of viable internalized Salmonella Typhimurium (plate count) was higher in the lower part than the level in the upper leafy part, especially when the leaves were contaminated with a high concentration of Salmonella (5 log CFU, P < 0.05), whereas the total internalized Salmonella Typhimurium (by qPCR) was higher in the upper part (P < 0.05) at the same contamination level. The discrepancy between these results suggests that most internalized Salmonella lost viability in the upper part but survived in the lower part. Water stress did not significantly change the extent of internalization in either location of green onion, whether detected via plate count or qPCR when the contamination occurred on the surface.

Root internalization, transport and in-planta survival of Salmonella enterica serovar Newport in sweet basil

It is now acknowledged that food-borne pathogens present in the irrigation water or soil can become associated with crop plants in the field, penetrate internal plant tissues via the root, translocate and survive inside plants. Only little information is available concerning interaction between enteric pathogens and plants. The present study evaluated the potential for contamination of the aromatic plant, sweet basil during cultivation, by Salmonella enterica serovar Newport. Root internalization was plant-age-dependent, with the highest susceptibility occurring at the beginning of the rapid growth phase of the root. Higher incidence of internalization was detected in vegetative than reproductive plant organs, pointing at bacterial transport in the transpiration stream. Internalized Salmonella survived only < 30 h in the phyllosphere. In contrast, survival of Salmonella on the leaf surface was much pronounced (at least 8 days), and the initial decay rate was lower at the abaxial (lower) compared with the adaxial (upper) side of the leaf. Although the experiments were conducted with high concentration of Salmonella unlikely to happen in the field, internalization occurred at a low frequency and in-planta survival was limited to less than 30 h. These findings imply that leaf surface contamination, rather than root internalization, may pose higher risk for human infection following consumption of contaminated basil.

Risk factors for microbial contamination in fruits and vegetables at the preharvest level: a systematic review

The objective of this study was to perform a systematic review of risk factors for contamination of fruits and vegetables with Listeria monocytogenes, Salmonella, and Escherichia coli O157:H7 at the preharvest level. Relevant studies were identified by searching six electronic databases: MEDLINE, EMBASE, CAB Abstracts, AGRIS, AGRICOLA, and FSTA, using the following thesaurus terms: L. monocytogenes, Salmonella, E. coli O157 AND fruit, vegetable. All search terms were exploded to find all related subheadings. To be eligible, studies had to be prospective controlled trials or observational studies at the preharvest level and had to show clear and sufficient information on the process in which the produce was contaminated. Of the 3,463 citations identified, 68 studies fulfilled the eligibility criteria. Most of these studies were on leafy greens and tomatoes. Six studies assessed produce contamination with respect to animal host-related risk factors, and 20 studies assessed contamination with respect to pathogen characteristics. Sixty-two studies assessed the association between produce contamination and factors related to produce, water, and soil, as well as local ecological conditions of the production location. While evaluations of many risk factors for preharvest-level produce contamination have been reported, the quality assessment of the reviewed studies confirmed the existence of solid evidence for only some of them, including growing produce on clay-type soil, the application of contaminated or non-pH-stabilized manure, and the use of spray irrigation with contaminated water, with a particular risk of contamination on the lower leaf surface. In conclusion, synthesis of the reviewed studies suggests that reducing microbial contamination of irrigation water and soil are the most effective targets for the prevention and control of produce contamination. Furthermore, this review provides an inventory of the evaluated risk factors, including those requiring more research.

Effect of repeated irrigation with water containing varying levels of total organic carbon on the persistence of Escherichia coli O157:H7 on baby spinach

The California lettuce and leafy greens industry has adopted the Leafy Greens Marketing Agreement (LGMA), which allows for 126 most-probable-number (MPN) Escherichia coli per 100 ml in irrigation water. Repeat irrigation of baby spinach plants with water containing E. coli O157:H7 and different levels of total organic carbon (TOC) was used to determine the epiphytic survival of E. coli O157:H7. Three irrigation treatments (0 ppm of TOC, 12 or 15 ppm of TOC, and 120 or 150 ppm of TOC) were prepared with bovine manure containing E. coli O157:H7 at either low (0 to 1 log CFU/100 ml) or high (5 to 6 log CFU/100 ml) populations, and sprayed onto baby spinach plants in growth chambers by using a fine-mist airbrush. MPN and direct plating techniques were used to determine the E. coli O157:H7 populations on the aerial plant tissue. Plants irrigated with high E. coli O157:H7 populations, regardless of TOC levels, showed a 3-log reduction within the first 24 h. Low levels of E. coli O157:H7 were observed for up to 16 days on all TOC treatments, ranging from 76.4 MPN per plant (day 1) to 0.40 MPN per plant (day 16). No viable cells were detected on spinach tissue 24 h after irrigation with water containing fewer than 126 CFU/100 ml E. coli O157:H7. Under growth chamber conditions in this study, E. coli O157:H7 populations in irrigation water that complies with the LGMA standards will not persist for more than 24 h when applied onto foliar surfaces of spinach plants.

Escherichia coli O157:H7: animal reservoir and sources of human infection

This review surveys the literature on carriage and transmission of enterohemorrhagic Escherichia coli (EHEC) O157:H7 in the context of virulence factors and sampling/culture technique. EHEC of the O157:H7 serotype are worldwide zoonotic pathogens responsible for the majority of severe cases of human EHEC disease. EHEC O157:H7 strains are carried primarily by healthy cattle and other ruminants, but most of the bovine strains are not transmitted to people, and do not exhibit virulence factors associated with human disease. Prevalence of EHEC O157:H7 is probably underestimated. Carriage of EHEC O157:H7 by individual animals is typically short-lived, but pen and farm prevalence of specific isolates may extend for months or years and some carriers, designated as supershedders, may harbor high intestinal numbers of the pathogen for extended periods. The prevalence of EHEC O157:H7 in cattle peaks in the summer and is higher in postweaned calves and heifers than in younger and older animals. Virulent strains of EHEC O157:H7 are rarely harbored by pigs or chickens, but are found in turkeys. The bacteria rarely occur in wildlife with the exception of deer and are only sporadically carried by domestic animals and synanthropic rodents and birds. EHEC O157:H7 occur in amphibian, fish, and invertebrate carriers, and can colonize plant surfaces and tissues via attachment mechanisms different from those mediating intestinal attachment. Strains of EHEC O157:H7 exhibit high genetic variability but typically a small number of genetic types predominate in groups of cattle and a farm environment. Transmission to people occurs primarily via ingestion of inadequately processed contaminated food or water and less frequently through contact with manure, animals, or infected people.