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

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.

Factors Required for Adhesion of Salmonella enterica Serovar Typhimurium to Lactuca sativa (Lettuce)

Salmonella enterica serovar Typhimurium is a major cause of foodborne gastroenteritis. Recent outbreaks of infections by S. enterica serovar Typhimurium are often associated with non-animal-related food, i.e., vegetables, fruits, herbs, sprouts, and nuts. One main problem related to the consumption of fresh produce is the minimal processing, especially for leafy green salads. In this study, we focused on butterhead lettuce (Lactuca sativa) to which S. enterica serovar Typhimurium adheres at higher rates compared to Valerianella locusta, resulting in prolonged persistence. Here, we systematically analyzed factors contributing to adhesion of S. enterica serovar Typhimurium to L. sativa leaves. Application of a reductionist, synthetic approach, including the controlled surface expression of specific adhesive structures of S. enterica serovar Typhimurium, one at a time, enabled the identification of relevant fimbrial and nonfimbrial adhesins, the O-antigen of lipopolysaccharide, the flagella, and chemotaxis being involved in binding to L. sativa leaves. The analyses revealed contributions of Lpf fimbriae, Sti fimbriae, autotransported adhesin MisL, T1SS-secreted BapA, intact lipopolysaccharide (LPS), and flagella-mediated motility to adhesion of S. enterica serovar Typhimurium to L. sativa leaves. In addition, we identified BapA as a potential adhesin involved in binding to V. locusta and L. sativa leaf surfaces. IMPORTANCE The number of produce-associated outbreaks by gastrointestinal pathogens is increasing and underlines the relevance to human health. The mechanisms involved in the colonization of, persistence on, and transmission by, fresh produce are poorly understood. Here, we investigated the contribution of adhesive factors of S. enterica serovar Typhimurium in the initial phase of plant colonization, i.e., the binding to the plant surface. We used the previously established reductionist, synthetic approach to identify factors that contribute to the surface binding of S. enterica serovar Typhimurium to leaves of L. sativa by expressing all known adhesive structures by remote control expression system.

Molecular characterization of endophytic and ectophytic plant growth promoting bacteria isolated from tomato plants (Solanum lycopersicum L.) grown in different soil types

Background

Successful rhizosphere colonization by plant growth promoting rhizobacteria (PGPR) is of crucial importance to perform the desired plant growth promoting activities. Since rhizocompetence is a dynamic process influenced by surrounding environmental conditions. In the present study, we hypothesized that bacterial isolates obtained from different tomato plant microhabitats (balk soil, rhizosphere, endorhiza, phyllosphere, and endoshoot) grown in different soils (sand, clay, and peat moss) will show different rhizocompetence abilities.

Results

To evaluate this hypothesis, bacterial isolates were obtained from different plant microhabitats and screened for their phosphate solubilizing and nitrogen fixing activates. BOX-PCR fingerprint profiles showed high genotypic diversity among the tested isolates and that same genotypes were shared between different soils and/or plant microhabitats. 16S rRNA gene sequences of 25 PGP isolates, representing different plant spheres and soil types, were affiliated to eight genera: Enterobacter, Paraburkholderia, Klebsiella, Bacillus, Paenibacillus, Stenotrophomonas, Pseudomonas, and Kosakonia. The rhizocompetence of each isolate was evaluated in the rhizosphere of tomato plants grown on a mixture of the three soils. Different genotypes of the same bacterial species displayed different rhizocompetence potentials. However, isolates obtained from the above-ground parts of the plant showed high rhizocompetence. In addition, biological control-related genes, ituD and srfC, were detected in the obtained spore forming bacterial isolates.

Conclusion

This study evaluates, for the first time, the relationship between plant microhabitat and the rhizocompetence ability in tomato rhizosphere. The results indicated that soil type and plant sphere can influence both the genotypic diversity and rhizocompetence ability of the same bacterial species. Bacterial isolates obtained in this study are promising to be used as an environmentally friendly substitution of chemical fertilizers.

Supplementary Information

The online version contains supplementary material available at 10.1186/s43141-022-00361-0.

Search for host defense markers uncovers an apple agglutination factor corresponding with fire blight resistance

Pathenogenesis-related (PR) proteins are extensively used as molecular markers to dissect the signaling cascades leading to plant defense responses. However, studies focusing on the biochemical or biological properties of these proteins remain rare. Here, we identify and characterize a class of apple (Malus domestica) PR proteins, named M. domestica AGGLUTININS (MdAGGs), belonging to the amaranthin-like lectin family. By combining molecular and biochemical approaches, we show that abundant production of MdAGGs in leaf tissues corresponds with enhanced resistance to the bacterium Erwinia amylovora, the causal agent of the disease fire blight. We also show that E. amylovora represses the expression of MdAGG genes by injecting the type 3 effector DspA/E into host cells and by secreting bacterial exopolysaccharides. Using a purified recombinant MdAGG, we show that the protein agglutinates E. amylovora cells in vitro and binds bacterial lipopolysaccharides at low pH, conditions reminiscent of the intercellular pH occurring in planta upon E. amylovora infection. We finally provide evidence that negatively charged polysaccharides, such as the free exopolysaccharide amylovoran progressively released by the bacteria, act as decoys relying on charge-charge interaction with the MdAGG to inhibit agglutination. Overall, our results suggest that the production of this particular class of PR proteins may contribute to apple innate immunity mechanisms active against E. amylovora.

Molecular determinants of surface colonisation in diarrhoeagenic Escherichia coli (DEC): from bacterial adhesion to biofilm formation

Escherichia coli is primarily known as a commensal colonising the gastrointestinal tract of infants very early in life but some strains being responsible for diarrhoea, which can be especially severe in young children. Intestinal pathogenic E. coli include six pathotypes of diarrhoeagenic E. coli (DEC), namely, the (i) enterotoxigenic E. coli, (ii) enteroaggregative E. coli, (iii) enteropathogenic E. coli, (iv) enterohemorragic E. coli, (v) enteroinvasive E. coli and (vi) diffusely adherent E. coli. Prior to human infection, DEC can be found in natural environments, animal reservoirs, food processing environments and contaminated food matrices. From an ecophysiological point of view, DEC thus deal with very different biotopes and biocoenoses all along the food chain. In this context, this review focuses on the wide range of surface molecular determinants acting as surface colonisation factors (SCFs) in DEC. In the first instance, SCFs can be broadly discriminated into (i) extracellular polysaccharides, (ii) extracellular DNA and (iii) surface proteins. Surface proteins constitute the most diverse group of SCFs broadly discriminated into (i) monomeric SCFs, such as autotransporter (AT) adhesins, inverted ATs, heat-resistant agglutinins or some moonlighting proteins, (ii) oligomeric SCFs, namely, the trimeric ATs and (iii) supramolecular SCFs, including flagella and numerous pili, e.g. the injectisome, type 4 pili, curli chaperone-usher pili or conjugative pili. This review also details the gene regulatory network of these numerous SCFs at the various stages as it occurs from pre-transcriptional to post-translocational levels, which remains to be fully elucidated in many cases.

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.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Efficacy of Acetic Acid or Chitosan for Reducing the Prevalence of Salmonella- and Escherichia coli O157:H7-Contaminated Leafy Green Plants in Field Systems

Outbreaks associated with fresh-cut leafy greens continue to occur despite efforts to implement horticultural practices that minimize introduction of enteric pathogens to the crop. The experimental trials in this study were designed to examine the efficacy of an acetic acid (AA)- and chitosan-based spray treatment, applied 1 day prior to harvest, for reducing the prevalence of Escherichia coli O157:H7 (O157) and Salmonella in field-grown leafy greens contaminated at levels detectable only through enrichment culture. Responses to the treatment solution were variable and depended on the type of leafy green (leafy lettuce, spinach, or cabbage), cultivar, pathogen, and AA concentration (0.3 to 0.7%). No significant differences in E. coli O157 prevalence were found for untreated and treated cabbage heads and spinach plants (P > 0.05). In contrast, treatment significantly affected Salmonella on 'Bravo F1' green cabbage and '7-Green' spinach (P < 0.05), with odds ratios of 2.2 and 3.3 for finding the pathogen on untreated versus treated greens, respectively. Salmonella was also 7.1 times more likely to be found on an untreated lettuce plant than on a lettuce plant sprayed with a 0.7% AA treatment solution (95% confidence interval [CI], 4.1 to 12.2; P < 0.0001). In studies addressing the efficacy of chitosan (0.1 or 0.3%), this chemical failed to reduce the prevalence of either pathogen on lettuce (P > 0.05). Similarly, spraying with 0.3% AA did not affect the prevalence of Salmonella on lettuce plants (P > 0.05); however, treatment solutions with 0.4% AA reduced the likelihood of detecting Salmonella in treated versus untreated plants by 6.6 times (95% CI, 2.1 to 20.9; P = 0.0007). After the lettuce was harvested and hand washed, consumers failed to distinguish either visually or organoleptically between untreated lettuce and lettuce sprayed with an acetic acid solution (P > 0.05). These results indicate that acetic acid could be used to reduce the microbiological risk of preharvest leafy greens.

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.

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

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