A comparative study on the effectiveness of chlorine dioxide gas, ozone gas and e-beam irradiation treatments for inactivation of pathogens inoculated onto tomato, cantaloupe and lettuce seeds

Effects of micro/nano-ozone bubble nutrient solutions on growth promotion and rhizosphere microbial community diversity in soilless cultivated lettuces

Due to its high efficacy as a wide-spectrum disinfectant and its potential for the degradation of pollutants and pesticides, ozone has broad application prospects in agricultural production. In this study, micro/nano bubble technology was applied to achieve a saturation state of bubble nutrient solution, including micro-nano oxygen (O2 group) and micro-nano ozone (O3 group) bubble nutrient solutions. The effects of these solutions on lettuce physiological indices as well as changes in the microbial community within the rhizosphere substrate were studied. The application of micro/nano (O2 and O3) bubble nutrient solutions to substrate-cultured lettuce plants increased the amount of dissolved oxygen in the nutrient solution, increased the lettuce yield, and elevated the net photosynthetic rate, conductance of H2O and intercellular carbon dioxide concentration of lettuce plants. Diversity analysis of the rhizosphere microbial community revealed that both the abundance and diversity of bacterial and fungal communities in the substrate increased after plant cultivation and decreased following treatment with micro/nanobubble nutrient solutions. RDA results showed that the microbial community in the S group was positively associated with EC, that in the CK and O2 groups exhibited a positive correlation with SC, and that in the O3 group displayed a positive correlation with CAT and POD. Overall, the implementation of micro/nanobubble generation technology in soilless substrates can effectively increase the lettuce growth and yield, and O3 had a more pronounced effect on lettuce yield and quality and the microbial community structure in the substrate than O2. Our study would provide a reference and theoretical basis for developing sustainable and green technology for promoting lettuce production and can be a promising alternative to conventional methods for improving crop yields.

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.

Incisional surface quality of electron-beam irradiated cornea-extracted lenticule for stromal keratophakia: high nJ-energy vs. low nJ-energy femtosecond laser

Introduction

Corneal lenticules can be utilized as an additive material for stromal keratophakia. However, following extraction, they must be reimplanted almost immediately or cryopreserved in lenticule banks. Electron-beam (E-beam) irradiated corneas permit room-temperature storage for up to 2 years, enabling keratophakia to be performed on demand. This study aims to compare the performance of high nano Joule (nJ)-energy (VisuMax) and low nJ-energy (FEMTO LDV) femtosecond laser systems on the thickness consistency and surface quality and collagen morphology of lenticules produced from fresh and E-beamed corneas.

Methods

A total of 24 lenticules with -6.00 dioptre power were cut in fresh human donor corneas and E-beamed corneas with VisuMax and FEMTO LDV. Before extraction, the thickness of the lenticules was measured with anterior segment-optical coherence tomography (AS-OCT). The incisional surface roughness of extracted lenticules was analyzed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). Multiphoton microscopy was then used to assess the surface collagen morphometry.

Results

The E-beamed lenticules that were cut using FEMTO LDV were significantly thicker than the fresh specimens as opposed to those created with VisuMax, which had a similar thickness as the fresh lenticules. On the vertex, they were ∼11% thicker than the fresh lenticules. The surface roughness (Rq) of E-beamed lenticules incised with FEMTO LDV did not differ significantly from the fresh lenticules. This contrasted with the VisuMax-fashioned lenticules, which showed notably smoother surfaces (∼36 and ∼20% lower Rq on anterior and posterior surfaces, respectively) on the E-beamed than the fresh lenticules. The FEMTO LDV induced less cumulative changes to the collagen morphology on the surfaces of both fresh and E-beamed lenticules than the VisuMax.

Conclusion

It has been previously demonstrated that the low nJ-energy FEMTO LDV produced a smoother cutting surface compared to high nJ-energy VisuMax in fresh lenticules. Here, we showed that this effect was also seen in the E-beamed lenticules. In addition, lower laser energy conferred fewer changes to the lenticular surface collagen morphology. The smaller disparity in surface cutting quality and collagen disturbances on the E-beamed lenticules could be beneficial for the early visual recovery of patients who undergo stromal keratophakia.

Generation, mechanisms, kinetics, and effects of gaseous chlorine dioxide in food preservation

Food preservation is a critical issue in ensuring food safety and quality. Growing concern around industrial pollution of food and demand for environmentally sustainable food has led to increased interest in developing effective and eco-friendly preservation techniques. Gaseous ClO2 has gained attention for its strong oxidizing properties, high efficacy in microorganism inactivation, and potential for preserving the attributes and nutritional quality of fresh food while avoiding the formation of toxic byproducts or unacceptable levels of residues. However, the widespread use of gaseous ClO2 in the food industry is limited by several challenges. These include large-scale generation, high cost and environmental considerations, a lack of understanding of its mechanism of action, and the need for mathematical models to predict inactivation kinetics. This review aims to provide an overview of the up-to-date research and application of gaseous ClO2 . It covers preparation methods, preservation mechanisms, and kinetic models that predict the sterilizing efficacy of gaseous ClO2 under different conditions. The impacts of gaseous ClO2 on the quality attributes of fresh produce and low-moisture foods, such as seeds, sprouts, and spices, are also summarized. Overall, gaseous ClO2 is a promising preservation approach, and future studies are needed to address the challenges in large-scale generation and environmental considerations and to develop standardized protocols and databases for safe and effective use in the food industry.

Effects of Plasma-Activated Water Treatment on the Inactivation of Microorganisms Present on Cherry Tomatoes and in Used Wash Solution

Herein, we investigated the potential of plasma-activated water (PAW) as a wash solution for the microbial decontamination of cherry tomatoes. We analyzed the efficacy of PAW as a bactericidal agent based on reactive species and pH. Immersion for 5 min in PAW15 (generated via plasma activation for 15 min) was determined as optimal for microbial decontamination of fresh produce. The decontamination efficacy of PAW15 exceeded those of mimic solutions with equivalent reactive species concentrations and pH (3.0 vs. 1.7 log reduction), suggesting that the entire range of plasma-derived reactive species participates in decontamination rather than a few reactive species. PAW15-washing treatment achieved reductions of 6.89 ± 0.36, 7.49 ± 0.40, and 5.60 ± 0.05 log₁₀ CFU/g in the counts of Bacillus cereus, Salmonella sp., and Escherichia coli O157:H7, respectively, inoculated on the surface of cherry tomatoes, with none of these strains detected in the wash solution. During 6 days of 25 °C storage post-washing, the counts of aerobic bacteria, yeasts, and molds were below the detection limit. However, PAW15 did not significantly affect the viability of RAW264.7 cells. These results demonstrate that PAW effectively inactivates microbes and foodborne pathogens on the surface of cherry tomatoes and in the wash solution. Thus, PAW could be used as an alternative wash solution in the fresh produce industry without cross-contamination during washing and environmental contamination by foodborne pathogens or potential risks to human health.

Ionizing Radiation: Effective Physical Agents for Economic Crop Seed Priming and the Underlying Physiological Mechanisms

To overcome various factors that limit crop production and to meet the growing demand for food by the increasing world population. Seed priming technology has been proposed, and it is considered to be a promising strategy for agricultural sciences and food technology. This technology helps to curtail the germination time, increase the seed vigor, improve the seedling establishment, and enhance the stress tolerance, all of which are conducive to improving the crop yield. Meanwhile, it can be used to reduce seed infection for better physiological or phytosanitary quality. Compared to conventional methods, such as the use of water or chemical-based agents, X-rays, gamma rays, electron beams, proton beams, and heavy ion beams have emerged as promising physics strategies for seed priming as they are time-saving, more effective, environmentally friendly, and there is a greater certainty for yield improvement. Ionizing radiation (IR) has certain biological advantages over other seed priming methods since it generates charged ions while penetrating through the target organisms, and it has enough energy to cause biological effects. However, before the wide utilization of ionizing priming methods in agriculture, extensive research is needed to explore their effects on seed priming and to focus on the underlying mechanism of them. Overall, this review aims to highlight the current understanding of ionizing priming methods and their applicability for promoting agroecological resilience and meeting the challenges of food crises nowadays.

Responses of Escherichia coli and Listeria monocytogenes to ozone treatment on non-host tomato: Efficacy of intervention and evidence of induced acclimation

Because of the continuous rise of foodborne illnesses caused by the consumption of raw fruits and vegetables, effective post-harvest anti-microbial strategies are necessary. The aim of this study was to evaluate the anti-microbial efficacy of ozone (O₃) against two common causes of fresh produce contamination, the Gram-negative Escherichia coli O157:H7 and Gram-positive Listeria monocytogenes, and to relate its effects to potential mechanisms of xenobiosis by transcriptional network modeling. The study on non-host tomato environment correlated the dose × time aspects of xenobiosis by examining the correlation between bacterial survival in terms of log-reduction and defense responses at the level of gene expression. In E. coli, low (1 μg O₃/g of fruit) and moderate (2 μg O₃/g of fruit) doses caused insignificant reduction in survival, while high dose (3 μg/g of fruit) caused significant reduction in survival in a time-dependent manner. In L. monocytogenes, moderate dose caused significant reduction even with short-duration exposure. Distinct responses to O₃ xenobiosis between E. coli and L. monocytogenes are likely related to differences in membrane and cytoplasmic structure and components. Transcriptome profiling by RNA-Seq showed that primary defenses in E. coli were attenuated after exposure to a low dose, while the responses at moderate dose were characterized by massive upregulation of pathogenesis and stress-related genes, which implied the activation of defense responses. More genes were downregulated during the first hour at high dose, with a large number of such genes getting significantly upregulated after 2 hr and 3 hr. This trend suggests that prolonged exposure led to potential adaptation. In contrast, massive downregulation of genes was observed in L. monocytogenes regardless of dose and exposure duration, implying a mechanism of defense distinct from that of E. coli. The nature of bacterial responses revealed by this study should guide the selection of xenobiotic agents for eliminating bacterial contamination on fresh produce without overlooking the potential risks of adaptation.

Inactivation of Escherichia coli O157:H7 on radish and cabbage seeds by combined treatments with gaseous chlorine dioxide and heat at high relative humidity

This study was done to develop a method to inactivate Escherichia coli O157:H7 on radish and cabbage seeds using simultaneous treatments with gaseous chlorine dioxide (ClO₂) and heat at high relative humidity (RH) without decreasing seeds' viability. Gaseous ClO₂ was spontaneously vaporized from a solution containing hydrochloric acid (HCl, 1 N) and sodium chlorite (NaClO₂, 100,000 ppm). Using a sealed container (1.8 L), an equation (y = 5687×, R² = 0.9948) based on the amount of gaseous ClO₂ generated from HCl-NaClO₂ solution at 60 °C and 85% RH was developed. When radish or cabbage seeds were exposed to gaseous ClO₂ at concentrations up to 3,000 ppm for 120 min, germination rates did not significantly decrease (P > 0.05). When seeds inoculated with E. coli O157:H7 were treated with 2,000 or 3,000 ppm of gaseous ClO₂ in an atmosphere with 85% RH at 60 °C, populations (6.8-6.9 log CFU/g) on both types of seeds were decreased to below the detection limit for enrichment (-0.5 log CFU/g) within 90 min. This study provides useful information for developing a decontamination method to control E. coli O157:H7 and perhaps other foodborne pathogens on plant seeds by simultaneous treatment with gaseous ClO₂ and heat at high RH.

Effect of Gaseous Ozone Process on Cantaloupe Melon Peel: Assessment of Quality and Antilisterial Indicators

Fruit waste parts, particularly peel, are abundant sources of bioactive compounds. To be included in the formulation of value-added foods, peel needs to be transformed and subjected to a preservation process. Therefore, this study seeks to assess the effect of ozone on the quality and antilisterial indicators of cantaloupe melon peel paste, aiming at obtaining a product with the potential to be used as a food additive. Ozone was bubbled during 30 and 60 min, and some physicochemical characteristics (soluble solids content, pH and colour), bioactive compounds (total phenolics, chlorophylls and vitamin C) and antioxidant activity were analysed. Peel was also inoculated with Listeria innocua, used as a treatment efficiency indicator. The results indicated that, although ozone negatively affected antioxidant activity, it positively influenced all bioactive compounds analysed. An L. innocua reduction of 1.2 log cycle was achieved after ozone exposure. Ozone should be exploited as a promising technology to assure the quality/safety of cantaloupe melon peel. Indeed, if melon peel is conveniently converted into a suitable form that can be used as a food ingredient, this will promote the valorisation of waste materials with the consequent reduction of industrial by-products and new perspectives for market opportunities.

Assessment of gaseous ozone treatment on Salmonella Typhimurium and Escherichia coli O157:H7 reductions in poultry litter

Poultry litter is used as soil amendment or organic fertilizer. While poultry litter is enriched with organic matter suitable for land, the presence of pathogens such as Salmonella in poultry litter is a concern. To investigate the effect of gaseous ozone on pathogen reductions in poultry litter, this study conducted a series of experiments that involved understanding of Salmonella Typhimurium and Escherichia coli O157:H7 inactivation at various doses of Ozone (O₃) in wet and dry poultry litter conditions. Previously, ozone treatment has been shown to disinfect the surface of foods and plant materials including fruits, juices, and wastewater, however, additional research are needed to better understand the impacts of ozone on treatment of soil amendments. Sanitizing methods capable of eliminating pathogens of soil amendments are crucial to mitigate disease outbreaks related with litter/manure-based fertilizers. In this study, a bench scale continuous ozone treatment system was designed to produce O₃ gas, with a range O₃ concentrations (7.15-132.46 mg·L^-1), monitor ozone concentrations continuously, and control the ozone exposure time (15 to 90 mins) to understand the effectiveness of O₃ in eliminating S. Typhimurium and E. coli O157:H7 in poultry litter. Results showed that 7.15 mg·L^-1 did not reduce the counts of S. Typhimurium until exposure to O₃ for 90 min. The O₃ concentrations of 43.26 ~ 132.46 mg·L^-1 exposure reduced the bacterial counts. Furthermore, the moisture content of poultry litter was found to be an influencing factor for pathogen reduction. The pathogen reduction rates were reduced when the moisture content was increased. At higher moisture content, high concentrations of O₃ (132.46 mg·L^-1) were needed for pathogen reductions. The moisture content of 30% or lower was found to be more effective for controlling pathogen levels in poultry litter. Our study demonstrates that gaseous O₃ treatment could be used as an additional decontamination technique to ensure the certain degree of microbiological safety of poultry litter based soil amendment.

Decontamination of Chia and Flax Seed Inoculated with Salmonella and Surrogate, Enterococcus faecium NRRL B-2354, Using a Peracetic Acid Sanitizing Solution: Antimicrobial Efficacy and Impact on Seed Functionality

Raw chia and flax seeds are increasingly associated with Salmonella contamination. However, intervention technologies for these seeds that maintain them in a raw state, without causing clumping because of mucilage production upon moisture exposure, are limited. In this study, a commercial ethanol and paracetic acid sanitizing solution meeting these criteria was evaluated for efficacy against Salmonella and Enterococcus faecium NRRL B-2354, a known Salmonella surrogate for thermal intervention technologies. Samples (100 g each) of chia and flax seeds ( n = 5) were inoculated with either a cocktail of Salmonella Newport, Senftenberg, Oranienburg, Saintpaul, Typhimurium DT104, and Cubana or E. faecium NRRL B-2354. After overnight acclimatization, samples were treated with 4 mL of sanitizing solution per sample and then held at ambient temperature (20 to 25°C) for 1 h before bacterial enumeration. Separate 1-kg-treated batches were evaluated for germination ability (4 replicates of 100-g samples), as well as nutrient content and rancidity ( n = 3), compared with untreated control. Following the posttreatment holding time, these batches were dried back to original moisture content at 70°C to evaporate residual sanitizing solution, thereby stopping treatment. The sanitizing solution was found to be an effective intervention method for chia and flax seeds, reducing Salmonella to below the level of detection by more than 4 and more than 5 average log CFU/g, respectively. Germination was not significantly affected ( P ≥ 0.05) for chia seed. For both seeds, nutrition and rancidity were not significantly affected ( P ≥ 0.05). Furthermore, E. faecium NRRL B-2354 was found to be an appropriate Salmonella surrogate for treatment of chia and flax seeds with this sanitizing solution, showing comparable but higher resistance to treatment with the sanitizing solution than the Salmonella cocktail.

Geobacillus and Bacillus Spore Inactivation by Low Energy Electron Beam Technology: Resistance and Influencing Factors

Low energy electron beam (LEEB) treatment is an emerging non-thermal technology that performs surface decontamination with a minimal influence on food quality. Bacterial spore resistance toward LEEB treatment and its influencing factors were investigated in this study. Spores from Geobacillus and Bacillus species were treated with a lab-scale LEEB at energy levels of 80 and 200 keV. The spore resistances were expressed as D-values (the radiation dose required for one log10 reduction at a given energy level) calculated from the linear regression of log10 reduction against absorbed dose of the sample. The results revealed that the spore inactivation efficiency by LEEB is comparable to that of other ionizing radiations and that the inactivation curves are mostly log10-linear at the investigated dose range (3.8 - 8.2 kGy at 80 keV; 6.0 - 9.8 kGy at 200 keV). The D-values obtained from the wildtype strains varied from 2.2 - 3.0 kGy at 80 keV, and from 2.2 - 3.1 kGy at 200 keV. Bacillus subtilis mutant spores lacking α/β-type small, acid-soluble spore proteins showed decreased D-values (1.3 kGy at 80 and 200 keV), indicating that spore DNA is one of the targets for LEEB spore inactivation. The results revealed that bacterial species, sporulation conditions and the treatment dose influence the spore LEEB inactivation. This finding indicates that for the application of this emerging technology, special attention should be paid to the choice of biological indicator, physiological state of the indicator and the processing settings. High spore inactivation efficiency supports the application of LEEB for the purpose of food surface decontamination. With its environmental, logistical, and economic advantages, LEEB can be a relevant technology for surface decontamination to deliver safe, minimally processed and additive-free food products.

Chloroxyanion Residue on Seeds and Sprouts after Chlorine Dioxide Sanitation of Alfalfa Seed

The effects of a 6-h chlorine dioxide sanitation of alfalfa seed (0, 50, 100, and 200 mg/kg seed) on total coliform bacteria, seed germination, and the presence of chlorate and perchlorate residues in seed rinse, seed soak, and alfalfa sprouts was determined. Chlorate residues in 20,000 mg/L calcium hypochlorite, commonly used to disinfect seed, were quantified. Chlorine dioxide treatment reduced (P < 0.05) total coliforms on seeds with no effect (P > 0.05) on germination. Dose-dependent sodium chlorate residues were present in seed rinse (4.1 to 31.2 μg/g seed) and soak (0.7 to 8.3 μg/g seed) waters, whereas chlorate residues were absent (LOQ 5 ng/g) in sprouts, except for 2 of 5 replicates from the high chlorine dioxide treatment. Copious chlorate residues were present (168 to 1260 mg/L) in freshly prepared 20,000 mg/L calcium hypochlorite solution, and storage at room temperature increased chlorate residues significantly (P < 0.01).

Ozone Sensitivity and Catalase Activity in Pigmented and Non-Pigmented Strains of Serratia Marcescens

Background:

Ozone exposure rapidly leads to bacterial death, making ozone an effective disinfectant in food industry and health care arena. However, microbial defenses may moderate this effect and play a role in the effective use of oxidizing agents for disinfection. Serratia marcescens is an opportunistic pathogen, expressing genes differentially during infection of a human host. A better understanding of regulatory systems that control expression of Serratia’s virulence genes and defenses is therefore valuable.

Objective:

Here, we investigated the role of pigmentation and catalase in Serratia marcescens on survival to ozone exposure.

Method:

Pigmented and non-pigmented strains of Serratia marcescens were cultured to exponential or stationary phase and exposed to 5 ppm of gaseous ozone for 2.5 – 10 minutes. Survival was calculated via plate counts. Catalase activity was measured photometrically and tolerance to hydrogen peroxide was assayed by disk-diffusion.

Results:

Exposure of S. marcescens to 5 ppm gaseous ozone kills > 90% of cells within 10 minutes in a time and concentration-dependent manner. Although pigmented Serratia (grown at 28°C) survived ozonation better than unpigmented Serratia (grown at 35°C), non-pigmented mutant strains of Serratia had similar ozone survival rates, catalase activity and H₂O₂ tolerance as wild type strains. Rather, ozone survival and catalase activity were elevated in 6 hour cultures compared to 48 hour cultures.

Conclusion:

Our studies did not bear out a role for prodigiosin in ozone survival. Rather, induction of oxidative stress responses during exponential growth increased both catalase activity and ozone survival in both pigmented and unpigmented S. marcescens.

Current Interventions for Controlling Pathogenic Escherichia coli

This review examined scientific reports and articles published from 2007 to 2016 regarding the major environmental sources of pathogenic Escherichia coli and the routes by which they enter the human gastrointestinal tract. The literature describes novel techniques used to combat pathogenic E. coli transmitted to humans from livestock and agricultural products, food-contact surfaces in processing environments, and food products themselves. Although prevention before contamination is always the best "intervention," many studies aim to identify novel chemical, physical, and biological techniques that inactivate or eliminate pathogenic E. coli cells from breeding livestock, growing crops, and manufactured food products. Such intervention strategies target each stage of the food chain from the perspective of "Farm to Table food safety" and aim to manage major reservoirs of pathogenic E. coli throughout the entire process. Issues related to, and recent trends in, food production must address not only the safety of the food itself but also the safety of those who consume it. Thus, research aims to discover new "natural" antimicrobial agents and to develop "multiple hurdle technology" or other novel technologies that preserve food quality. In addition, this review examines the practical application of recent technologies from the perspective of product quality and safety. It provides comprehensive insight into intervention measures used to ensure food safety, specifically those aimed at pathogenic E. coli.

Differential Inactivation of Fungal Spores in Water and on Seeds by Ozone and Arc Discharge Plasma

Seed sterilization is essential for preventing seed borne fungal diseases. Sterilization tools based on physical technologies have recently received much attention. However, available information is very limited in terms of efficiency, safety, and mode of action. In this study, we have examined antifungal activity of ozone and arc discharge plasma, potential tools for seed sterilization. In our results, ozone and arc discharge plasma have shown differential antifungal effects, depending on the environment associated with fungal spores (freely submerged in water or infected seeds). Ozone inactivates Fusarium fujikuroi (fungus causing rice bakanae disease) spores submerged in water more efficiently than arc discharge plasma. However, fungal spores associated with or infecting rice seeds are more effectively deactivated by arc discharge plasma. ROS generated in water by ozone may function as a powerful fungicidal factor. On the other hand, shockwave generated from arc discharge plasma may have greatly contributed to antifungal effects on fungus associated with rice seeds. In support of this notion, addition of ultrasonic wave in ozone generating water has greatly increased the efficiency of seed disinfection.

Efficacy of various sanitizers against Salmonella during simulated commercial packing of tomatoes

Chemical sanitizers are usually added to dump tank water to minimize cross-contamination during tomato packing. However, the efficacy of sanitizers continues to be questioned. This study assessed the ability of six commonly used sanitizers (40 ppm of peroxyacetic acid, 40 ppm of mixed peracid, 40 ppm of available chlorine alone or acidified to pH 6.0 with citric acid or T-128, and electrolyzed water containing 40 ppm of available chlorine at pH 6.7) to reduce Salmonella on tomatoes, in wash water, and on equipment surfaces using a pilot-scale processing line. Red round tomatoes (11.3 kg) were dip inoculated to contain Salmonella at ∼6 log CFU/g, air dried for 2 h, treated for 2 min in a 3.3-m-long dump tank and then dried on a roller conveyor, with sanitizer-free water serving as the control. Tomato and water samples were collected at 15-s intervals during washing with additional dump tank, water tank, and roller conveyor surface samples collected after washing. All samples were appropriately neutralized, diluted, and surface plated on Trypticase soy agar containing 0.6% yeast extract, 0.05% ferric ammonium citrate, and 0.03% sodium thiosulfate with or without membrane filtration to enumerate Salmonella. All six sanitizer treatments were more efficacious than the water control (P ≤ 0.05), with chlorine plus citric acid yielding the greatest Salmonella reduction on tomatoes (3.1 log CFU/g). After processing, all sanitizer wash solutions contained significantly lower (P ≤ 0.05) levels of Salmonella than the water control (3.0 log CFU/ml). The four chlorine-based sanitizer treatments yielded significantly lower Salmonella populations (P ≤ 0.05) in the wash solution compared with peroxyacetic acid and mixed peracid. After processing with sanitizers, Salmonella populations decreased to nondetectable levels (<0.2 log CFU/100 cm(2) ) on the equipment surfaces.

Leveraging management strategies for seedborne plant diseases to reduce Salmonella enterica Serovar Typhimurium incidence on tomato seed and seedlings

Tomatoes have been linked to many outbreaks of salmonellosis over the last decade, but the routes of contamination have yet to be discerned. Many phytopathogens of tomato are seedborne and are effectively managed using seed sanitizers. Seed sanitizers effective against bacterial phytopathogens were evaluated for their efficacy in killing bioluminescent Salmonella enterica serovar Typhimurium strain SeT-A14 on tomato seed infested with moderately high and high levels of pathogen. SeT-A14 incidence on seedlings produced from contaminated seed following sanitation was also determined. At a moderately high infestation rate (40%), SeT-A14 was eradicated on seed sanitized with 1.2% sodium hypochlorite (NaClO) mixed with 0.03% surfactant for 2 min, hydrochloric acid (HCl) for 30 min, and trichloromelamine for 2 min. At a higher infestation rate (94%), only NaClO and HCl were effective in eradicating SeT-A14 from the seed. At both infestation rates, 2% Virkon-S for 15 min significantly reduced SeT-A14 incidence compared with the nontreated infested controls but did not eradicate the pathogen. Hot water, a commonly used sanitizer for managing seedborne bacterial plant diseases, significantly reduced SeT-A14 on heavily infested seed, but incidence was still moderate at 17.5%. On seedlings produced from moderately highly infested seed, SeT-A14 was not detected using RapidChek Salmonella test strips. Using heavily infested seed, SeT-A14 was detected with the test strips in one of four pooled samples of 14-day-old seedlings produced from nonsanitized seed and from seed sanitized with hot water and trichloromelamine. However, bioluminescence was not observed on 14-day-old seedlings. To our knowledge, this is the first report that provides evidence that S. enterica serovar Typhimurium can be seed transmitted and can lead to the contamination of tomato seedlings. In addition to eliminating important bacterial phytopathogens from tomato seed, NaClO or HCl may mitigate the risk of Salmonella seedling contamination.

Long-term survival of Escherichia coli O157:H7 and Salmonella enterica on butterhead lettuce seeds, and their subsequent survival and growth on the seedlings

The long-term survival of enteric pathogens on butterhead lettuce seeds, and their subsequent survival and growth on seedlings were investigated. Lettuce seeds were inoculated at a high level with two Salmonella enterica and two Escherichia coli O157:H7 strains each (±8 log₁₀ CFU/g seed) and the survival of the pathogens was monitored over two years using standard plating techniques on selective medium. The Salmonella strains (serovars Typhimurium and Thompson) survived significantly better on the seeds than the E. coli O157:H7 strains (MB3885 and NCTC12900). When individual seeds were tested two years after inoculation, Salmonella was recovered from each individual seed, whereas E. coli O157:H7 only from 4% to 14% of the seeds, depending on the recovery method. When contaminated stored seeds were germinated and the seedlings examined for presence of the pathogens, it was clear that both pathogens were able to proliferate on the seedlings. Pathogen counts up to 5.92 log₁₀ CFU and 4.41 log₁₀ CFU per positive seedling were observed for Salmonella and E. coli O157:H7, respectively. Our study not only confirms the long-term survival of enteric pathogens on seeds but also shows that the pathogens maintain their ability to resuscitate and proliferate on the seedlings. Seeds or seedlings should be considered as contamination sources for the cultivation of leafy vegetables such as butterhead lettuce grown in greenhouses.