Listeria monocytogenesGrowth Dynamics on Produce: A Review of the Available Data for Predictive Modeling

The Gamma concept approach as a tool to predict fresh produce supporting or not the growth of L. monocytogenes

Challenge tests are commonly employed to evaluate the growth behavior of L. monocytogenes in food matrices; they are known for being expensive and time-consuming. An alternative could be the use of predictive models to forecast microbial behavior under different conditions. In this study, the growth behavior of L. monocytogenes in different fresh produce was evaluated using a predictive model based on the Gamma concept considering pH, water activity (aw), and temperature as input factors. An extensive literature search resulted in a total of 105 research articles selected to collect growth/no growth behavior data of L. monocytogenes. Up to 808 L. monocytogenes behavior values and physicochemical characteristics were extracted for different fruits and vegetables. The predictive performance of the model as a tool for identifying the produce commodities supporting the growth of L. monocytogenes was proved by comparing with the experimental data collected from the literature. The model provided satisfactory predictions on the behavior of L. monocytogenes in vegetables (>80% agreement with experimental observations). For leafy greens, a 90% agreement was achieved. In contrast, the performance of the Gamma model was less satisfactory for fruits, as it tends to overestimate the potential of acid commodities to inhibit the growth of L. monocytogenes.

Dynamics of Listeriamonocytogenes and Salmonella enterica on Cooked Vegetables During Storage

Fresh vegetables have been linked to multiple foodborne outbreaks in the U.S., with Listeria monocytogenes and Salmonella enterica identified as leading causes. Beyond raw vegetables, cooked vegetables can also pose food safety concerns due to improper cooking temperature and time combinations or postcooking contamination. Cooked vegetables, having had their native microbiota reduced through heat inactivation, might provide an environment that favors the growth of pathogens due to diminished microbial competition. While the risks associated with raw vegetables are recognized, the survival and growth of pathogens on cooked vegetables remain inadequately studied. This study investigated the growth kinetics of both L. monocytogenes and S. enterica on various cooked vegetables (carrot, corn, onions, green bell pepper, and potato). Vegetables were cooked at 177°C until the internal temperature reached 90°C and then cooled to 5°C. Cooled vegetables were inoculated with a four-strain cocktail of either L. monocytogenes or S. enterica at 3 log CFU/g, then stored at different temperatures (5, 10, or 25°C) for up to 7 days. Both pathogens survived on all vegetables when stored at 5°C. At 10°C, both pathogens proliferated on all vegetables, with the exception of L. monocytogenes on pepper. At 25°C, the highest growth rates were observed by both pathogens on carrot (5.55 ± 0.22 and 6.42 ± 0.23 log CFU/g/d for L. monocytogenes and S. enterica, respectively). S. enterica displayed higher growth rates at 25°C compared to L. monocytogenes on all vegetables. Overall, these results bridge the knowledge gap concerning the growth kinetics of both S. enterica and L. monocytogenes on various cooked vegetables, offering insights to further enhance food safety.

Listeriosis Risk Model for Cancer Patients Who Consume Ready-to-Eat Salad

The foodborne pathogen Listeria monocytogenes generally infects immunocompromised individuals, such as cancer patients, more frequently and with higher morbidity and mortality than the general population. Because of the anticipated risk associated with L. monocytogenes and other pathogens in produce, immunocompromised individuals are often placed on neutropenic diets that exclude fresh produce, though these risks have not been quantified. Therefore, this study developed a data-driven risk model for listeriosis in cancer patients who consume ready-to-eat (RTE) salads, consisting of leafy greens, cucumbers, and tomatoes, as influenced by kitchen-scale treatments and storage practices. Monte Carlo simulations were used to model the risk of invasive listeriosis during one chemotherapy cycle. Refrigerating all salad components decreased median risk by approximately one-half log. For refrigerated salads with no treatment, the predicted median risk was ≤ 4.3 × 10^-08. When salad ingredients were surface blanched with greens rinsed, the predicted risk decreased to 5.4 × 10^-10. Predicted risk was lowest (1.4 × 10^-13) for a blanched "salad" consisting of solely cucumbers and tomatoes. Interestingly, rinsing, as recommended by FDA only decreased median risk by 1 log. A sensitivity analysis revealed that the highly variable dose-response parameter k strongly influenced risk, indicating that reducing uncertainty in this variable may improve model accuracy. Overall, this study demonstrates that kitchen-scale pathogen reduction approaches have high risk-reduction efficacy and could be considered as an alternative to diets that exclude produce when making risk management decisions.

Growth and inactivation of Listeria monocytogenes in sterile extracts of fruits and vegetables: Impact of the intrinsic factors pH, sugar and organic acid content

Intrinsic characteristics of fresh produce, such as pH, water activity, acid content and nutrient availability are critical factors in determining the survival and growth of Listeria monocytogenes (Lm). In this study, sterile fresh produce juice was used to analyze Lm growth potential among 14 different commodities and to identify physicochemical characteristics in those juices that affect Lm growth. Significant growth of Lm was observed in juices with pH ≥5.6 and low acidity (0.04-0.07 % titratable acidity (TA)) (cantaloupe, carrot, celery, green pepper, parsley, and romaine lettuce), slight reduction of Lm was observed in juices with pH 4.1 (tomato) and pH 3.9 (mango), and no Lm counts were recovered from juices with pH ≤3.8 and high acidity (0.28-1.17 % TA) (apple, blueberry, grape, peach, and pineapple). Although these acidic fruit juices possessed a high sugar content, the pH and acidity of produce juice seemed to be the primary determinants for Lm growth. The neutralization of acidic juices (i.e., Fuji and Gala apple, blueberry, grape, mango, pineapple, peach, and tomato) enabled Lm growth at 37 °C in all juices except for Gala apple and peach. Strong decline in Lm populations in Gala apple, grape and peach juices might be linked to sensitivity to organic acids, such as malic acid. Furthermore, Lm populations significantly decreased in pH-neutral (7.6) cauliflower juice, suggesting that potential antilisterial substances may play a role in Lm decline in cauliflower juice.

Growth Kinetics of Listeria monocytogenes on Cut Red Cabbage

ABSTRACT

Listeria monocytogenes is a foodborne pathogen associated with fresh produce vectors such as leafy greens. Recent outbreaks and recalls associated with red cabbage-containing salads have brought attention to this food commodity. Although data on the proliferation of L. monocytogenes are available for different varieties of whole and cut white cabbage, no information is available on the fate of this pathogen on red cabbage. The aim of this study was therefore to explore the survival of L. monocytogenes on cut red cabbage (Brassica oleracea L. var. capitata f. rubra) during storage at different temperatures. Red cabbage was cut into pieces (5 by 4 cm) and spot inoculated with a six-strain cocktail of rifampin-resistant L. monocytogenes, resulting in an initial inoculation level of 4 log CFU/g. Samples were stored at 5 or 10°C for 14 days or at 25°C for 7 days. At intervals during storage, L. monocytogenes was enumerated by homogenization of cabbage with Butterfield's phosphate buffer, and serial dilutions were plated onto brain heart infusion agar supplemented with rifampin. No growth of L. monocytogenes was observed on cut red cabbage during storage at 5°C, and only minimal proliferation was observed at the higher temperatures. Significant population increases of 0.58 and 1.07 log CFU/g were determined after 3 days of storage at 10 and 25°C, respectively; however, a significant decrease of 0.77 log CFU/g from 3 to 14 days was also observed at 10°C. The modeled growth rates for L. monocytogenes on cut red cabbage stored at 5 and 10°C were 0.11 ± 0.03 and 0.27 ± 0.07 log CFU/g/day, with calculated times to a 1-log CFU/g increase of 9.51 and 3.70 days, respectively; however, L. monocytogenes did not achieve a 1-log increase at either temperature in this study. At 25°C, the modeled growth rate of L. monocytogenes on cut red cabbage was 1.15 ± 0.36 log CFU/g, leading to a calculated and an observed 1-log increase in 0.87 and 3.00 days, respectively. Results from this study aid in understanding the fate of L. monocytogenes on cut red cabbage during storage at different temperatures.

HIGHLIGHTS

Initial and Final Cell Concentrations Significantly Influence the Maximum Growth Rate of Listeria monocytogenes in Published Literature Data for Whole Intact Fresh Produce

ABSTRACT

Listeria monocytogenes has shown the ability to grow on fresh uncut produce; however, the factors that control growth are not well understood. Peer-reviewed journal articles (n = 29) meeting the inclusion criteria and related to the growth of L. monocytogenes on fresh produce were found through university library databases and Google Scholar searches. Growth models were fit to each of the extracted 130 data sets to estimate log CFU per day rates of growth by using the DMFit tool. Multiple linear stepwise regression models for factors influencing growth rate were developed using R software. Factors included were temperature, nutrient level of inoculation buffer, initial cell concentration, final cell concentration, inoculation method, container permeability, and surface characteristics. The full model produced adjusted R2, Akaike information criterion, and root mean square error values of 0.41, 488, and 1.61, respectively. Stepwise regression resulted in a reduced model with parameters for incubation temperature, inoculation buffer type, initial and final cell concentrations, container characteristics, and produce surface characteristics. Model fit statistics improved slightly in the reduced model. A further reduced three-parameter model included storage temperature and initial and final cell concentrations, with interaction terms. This three-parameter model had adjusted R2, Akaike information criterion, and root mean square error values of 0.66, 417, and 1.24, respectively. Incubation temperature (P = 1.00E-09) initial cell concentration (P = 3.05E-12), and final cell concentration (P = 4.17E-09) all had highly significant effects on maximum growth rate. Our findings show the importance of inoculum concentration and produce microbial carrying capacity on the estimated growth rate and highlight the overall importance that temperature has on growth rate. Future experiments should consider initial inoculum concentration carefully when conducting growth studies for L. monocytogenes on whole produce.

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Listeria monocytogenes Survival on Peaches and Nectarines under Conditions Simulating Commercial Stone-Fruit Packinghouse Operations

Recent recalls of stone fruit due to potential Listeria contamination and associated foodborne outbreaks highlight the risk for pathogen transmission through stone-fruit consumption. Particularly, surface contamination of fruits increases the risk for cross-contamination of produce during processing and storage. This highlights the need for quality control in stone fruits intended for consumption. To develop effective food safety practices, it is essential to determine the critical factors during stone-fruit processing that influence Listeria survival. Therefore, this study evaluated the ability of Listeria to survive on peaches and nectarines under simulated stone-fruit loading and staging, waxing and fungicide application and storage conditions. The results of our study indicate that current stone-fruit handling conditions do not favor Listeria growth. However, once fruit is contaminated, Listeria can survive on the fruit surface in significant numbers under current processing conditions. Therefore, there is a need to develop and implement preventive controls at the stone-fruit packinghouse to prevent Listeria contamination and deter pathogen persistence.

Anti-listerial properties of chemical constituents of Eruca sativa (rocket salad): From industrial observation to in vitro activity

The frequency of foodborne outbreaks epidemiologically associated with Listeria monocytogenes in fresh produce has increased in recent years. Although L. monocytogenes may be transferred from the environment to vegetables during farming, contamination of food products most commonly occurs in food processing facilities, where L. monocytogenes has the ability to establish and persist on processing equipment. The current study was undertaken to collect data on the occurrence of L. monocytogenes and the identity of the endogenous microbiota in a fresh produce processing facility, for which information has remained scarce. L. monocytogenes was not detected in the facility. Experiments simulating conditions in the processing environment were performed, including examination of bacterial growth in nutrients based on vegetables (salad juice) compared to in other types of nutrients (fish, meat). Results showed that the endogenous microbiota (dominated by Pseudomonas) grew well in iceberg lettuce and rocket salad juice at low temperatures, while growth inhibition of L. monocytogenes was observed, particularly in rocket salad juice. The anti-listerial activity in rocket salad juice was retained in a polar chromatographic fraction containing several metabolites. Characterization of this active fraction, using LC-MS/MS, led to identification of 19 compounds including nucleosides and amino acids. Further work is necessary to determine the molecular mechanism responsible for the inhibitory activity of rocket salad constituents. The study nevertheless suggests that the available nutrients, as well as a low temperature (3 °C) and the in-house bacterial flora, may influence the prevalence of L. monocytogenes in fresh produce processing facilities.

ComBase Models Are Valid for Predicting Fate of Listeria monocytogenes on 10 Whole Intact Raw Fruits and Vegetables

ABSTRACT

Listeria monocytogenes was associated with more than 60 produce recalls, including tomato, cherry, broccoli, lemon, and lime, between 2017 and 2020. This study describes the effects of temperature, time, and food substrate as factors influencing L. monocytogenes behavior on whole intact raw fruits and vegetables. Ten intact whole fruit and vegetable commodities were chosen based on data gaps identified in a systematic literature review. Produce investigated belong to major commodity families: Ericaceae (blackberry, raspberry, and blueberry), Rutaceae (lemon and mandarin orange), Roseaceae (sweet cherry), Solanaceae (tomato), Brassaceae (cauliflower and broccoli), and Apiaceae (carrot). A cocktail of five L. monocytogenes strains that included clinical, food, or environmental isolates linked to foodborne outbreaks was used to inoculate intact whole fruits and vegetables. Samples were incubated at 2, 12, 22, 30, and 35°C with relative humidities matched to typical real-world conditions. Foods were sampled (n = 6) for up to 28 days, depending on temperature. Growth and decline rates were estimated using DMFit, an Excel add-in. Growth rates were compared with ComBase modeling predictions for L. monocytogenes. Almost every experiment showed initial growth, followed by subsequent decline. L. monocytogenes was able to grow on the whole intact surface of all produce tested, except for carrot. The 10 produce commodities supported growth of L. monocytogenes at 22 and 35°C. Growth and survival at 2 and 12°C varied by produce commodity. The standard deviation of the square root growth and decline rates showed significantly larger variability in both growth and decline rates within replicates as temperature increased. When L. monocytogenes growth occurred, it was conservatively modeled by ComBase Predictor, and growth was generally followed by decreases in concentration. This research will assist in understanding the risks of foodborne disease outbreaks and recalls associated with L. monocytogenes on fresh whole produce.

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Guidance on date marking and related food information: part 1 (date marking)

A risk-based approach was developed to be followed by food business operators (FBO) when deciding on the type of date marking (i.e. 'best before' date or 'use by' date), setting of shelf-life (i.e. time) and the related information on the label to ensure food safety. The decision on the type of date marking needs to be taken on a product-by-product basis, considering the relevant hazards, product characteristics, processing and storage conditions. The hazard identification is food product-specific and should consider pathogenic microorganisms capable of growing in prepacked temperature-controlled foods under reasonably foreseeable conditions. The intrinsic (e.g. pH and aw), extrinsic (e.g. temperature and gas atmosphere) and implicit (e.g. interactions with competing background microbiota) factors of the food determine which pathogenic and spoilage microorganisms can grow in the food during storage until consumption. A decision tree was developed to assist FBOs in deciding the type of date marking for a certain food product. When setting the shelf-life, the FBO needs to consider reasonably foreseeable conditions of distribution, storage and use of the food. Key steps of a case-by-case procedure to determine and validate the shelf-life period are: (i) identification of the relevant pathogenic/spoilage microorganism and its initial level, (ii) characterisation of the factors of the food affecting the growth behaviour and (iii) assessment of the growth behaviour of the pathogenic/spoilage microorganism in the food product during storage until consumption. Due to the variability between food products and consumer habits, it was not appropriate to present indicative time limits for food donated or marketed past the 'best before' date. Recommendations were provided relating to training activities and support, using 'reasonably foreseeable conditions', collecting time-temperature data during distribution, retail and domestic storage of foods and developing Appropriate Levels of Protection and/or Food Safety Objectives for food-pathogen combinations.

Fate of Listeria monocytogenes on Broccoli and Cauliflower at Different Storage Temperatures

ABSTRACT

Understanding a food's ability to support the growth and/or survival of a pathogen throughout the supply chain is essential to minimizing large-scale contamination events. The purpose of this study was to examine the behavior (growth and/or survival) of Listeria monocytogenes on broccoli and cauliflower florets stored at different postharvest temperatures utilized along the supply chain. Broccoli and cauliflower samples were inoculated with L. monocytogenes at approximately 3 log CFU/g and stored at 23 ± 2, 12 ± 2, 4 ± 2, and -18 ± 2°C. Samples were evaluated for L. monocytogenes levels after 0, 0.167 (4 h), 1, 2, 3, and 4 days at 23 ± 2°C; 0, 0.167, 1, 2, 3, 4, 7, 10, and 14 days at 12 ± 2°C; 0, 0.167, 1, 2, 3, 4, 7, 10, 14, 21, and 28 days at 4 ± 2°C; and 0, 1, 7, 28, 56, 84, 112, 140, and 168 days at -18 ± 2°C. L. monocytogenes populations were determined by plating samples onto tryptic soy agar and modified Oxford agar supplemented with nalidixic acid. Broccoli and cauliflower supported the growth of L. monocytogenes at 23, 12, and 4°C, and higher growth rates were observed at higher temperatures. Populations of L. monocytogenes on broccoli and cauliflower samples significantly increased within 1 day at 23°C (by 1.6 and 2.0 log CFU/g, respectively) (P ≤ 0.05). At 12°C, populations of L. monocytogenes on broccoli and cauliflower samples significantly increased over 14 days by 1.4 and 1.9 log CFU/g, respectively (P ≤ 0.05). No significant difference over time was observed in L. monocytogenes populations on broccoli and cauliflower samples held under refrigeration until populations began to grow by day 10 in both commodities (P > 0.05). Under frozen storage (-18°C), populations of L. monocytogenes survived on broccoli and cauliflower at least up to 168 days. Storage of broccoli and cauliflower at lower temperatures can minimize L. monocytogenes growth potential; growth rates were lower at 4°C than at 12 and 23°C.

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Growth and Survival of Listeria monocytogenes on Intact Fruit and Vegetable Surfaces during Postharvest Handling: A Systematic Literature Review

Listeria monocytogenes may be present in produce-associated environments (e.g., fields, packing houses); thus, understanding its growth and survival on intact, whole produce is of critical importance. The goal of this study was to identify and characterize published data on the growth and/or survival of L. monocytogenes on intact fruit and vegetable surfaces. Relevant studies were identified by searching seven electronic databases: AGRICOLA, CAB Abstracts, Center for Produce Safety funded research project final reports, FST Abstracts, Google Scholar, PubMed, and Web of Science. Searches were conducted using the following terms: Listeria monocytogenes, produce, growth, and survival. Search terms were also modified and "exploded" to find all related subheadings. Included studies had to be prospective, describe methodology (e.g., inoculation method), outline experimental parameters, and provide quantitative growth and/or survival data. Studies were not included if methods were unclear or inappropriate, or if produce was cut, processed, or otherwise treated. Of 3,459 identified citations, 88 were reviewed in full and 29 studies met the inclusion criteria. Included studies represented 21 commodities, with the majority of studies focusing on melons, leafy greens, berries, or sprouts. Synthesis of the reviewed studies suggests L. monocytogenes growth and survival on intact produce surfaces differ substantially by commodity. Parameters such as temperature and produce surface characteristics had a considerable effect on L. monocytogenes growth and survival dynamics. This review provides an inventory of the current data on L. monocytogenes growth and/or survival on intact produce surfaces. Identification of which intact produce commodities support L. monocytogenes growth and/or survival at various conditions observed along the supply chain will assist the industry in managing L. monocytogenes contamination risk.

Aerated Steam Sanitization of Whole Fresh Cantaloupes Reduces and Controls Rind-Associated Listeria but Enhances Fruit Susceptibility to Secondary Colonization

Recent bacterial illnesses and outbreaks associated with the consumption of fresh and fresh-cut fruit and vegetables emphasize the need to supply produce that is microbiologically safe while retaining its quality and nutrient value. We assessed the capacity of aerated steam to reduce initial levels and control the posttreatment proliferation of a 4-strain mixture of Listeria innocua, a surrogate for L. monocytogenes, and microflora native to the rind of whole cantaloupes. Studies were conducted at the pilot-scale level by passing deliberately contaminated melons through a prototype stainless-steel, continuous-feed heating device. Exposure for 240 s to aerated steam heated to 85 °C achieved a mean reduction in surface-inoculated L. innocua of 3.9 ± 0.6 log₁₀ CFU/cm² (n = 3) and decreased background microorganisms (yeast, moulds, and coliforms) to undetectable levels. No significant outgrowth of surviving L. innocua or yeast and moulds was observed on heat-treated melons during their storage at 4, 7, and 10 °C for 14 days. Treated fruit continued to respire. Although rind quality was altered, edible fleshy portions remained largely unaffected. Cantaloupe inoculated with L. innocua subsequent to its exposure to aerated steam provided a suitable environment for surrogate growth (mean 3.3 log₁₀ increase in rind density over 10 days at 7 °C), whereas its proliferation was restricted on nonheated cantaloupe (mean 0.7 log₁₀ increase). Steam sanitization provides an effective means for the control of pathogen and spoilage organisms, but the proliferation of surrogate organisms on heated cantaloupes raises concern regarding the impact of postprocessing contamination on consumer health risk.

PRACTICAL APPLICATION

Water vapor (steam) at a high temperature can be used to sanitize the surface of fresh, whole cantaloupe melons in a continuous-feed manner. Both Listeria bacteria and spoilage organisms are markedly reduced from initial levels and survivor outgrowth severely restricted during subsequent refrigerated storage. This approach to microorganism control is likely most applicable in situations where rinds and flesh are to be separated immediately via further processing.

Oral exposure to Listeria monocytogenes in aged IL-17RKO mice: A possible murine model to study listeriosis in susceptible populations

Foodborne Listeria monocytogenes (LM) is a cause of serious illness and death in the US. The case-fatality rate of invasive LM infection in the elderly population is >50%. The goal of this study is to establish a murine model of oral LM infection that can be used as a surrogate for human foodborne listeriosis in the geriatric population. Adult C57BL/6 (wild-type, WT) and adult or old IL17R-KO (knock-out) mice were gavaged with a murinized LM strain (Lmo-InlA^m) and monitored for body-weight loss and survivability. Tissues were collected and assayed for bacterial burden, histology, and cytokine responses. When compared to WT mice, adult IL17R-KO mice are more susceptible to LM infection and showed increased LM burden and tissue pathology and a higher mortality rate. Older LM-infected KO-mice lost significantly (p < 0.02, ANOVA) more body-weight and had a higher bacterial burden in the liver (p = 0.03) and spleen as compared to adult mice. Uninfected, aged KO-mice showed a higher baseline pro-inflammatory response when compared to uninfected adult-KO mice. After infection, the pro-inflammatory cytokine, IFN-γ, mRNA in the liver was higher in the adult mice as compared to the old mice. The anti-inflammatory cytokine, IL-10, mRNA and regulatory T-cells (CD4⁺CD25^+h or CD4⁺Foxp3⁺) cells in the aged mice increased significantly after infection as compared to adult mice. Expression of the T-cell activation marker, CD25 (IL-2Rα) in the aged mice did not increase significantly over baseline. These data suggest that aged IL17R-KO mice can be used as an in vivo model to study oral listeriosis and that aged mice are more susceptible to LM infection due to dysregulation of pro- and anti-inflammatory responses compared to adult mice, resulting in a protracted clearance of the infection.

Survival and High-Hydrostatic Pressure Inactivation of Foodborne Pathogens in Salmorejo, a Traditional Ready-to-Eat Food

Salmorejo is a traditional tomato-based creamy product. Because salmorejo is not heat-processed, there is a risk of contamination with foodborne pathogens from raw materials. Even though bacterial growth in salmorejo is strongly inhibited because of its acidic pH (close to 3.9), the growth and survival of 3 foodborne pathogens in this food has not been studied before. In this study, 3 cocktails consisting of Escherichia coli O157, Salmonella enterica serovar Enteritidis, and Listeria monocytogenes strains were inoculated in freshly prepared salmorejo. The food was treated by high hydrostatic pressure (HHP) at 400, 500, or 600 MPa for 8 min, or left untreated, and stored at 4 °C for 30 d. Viable cell counts were determined on selective media and also by the triple-layer agar method in order to detect sublethally injured cells. In control samples, L. monocytogenes viable cells decreased by 2.4 log cycles at day 7 and were undetectable by day 15. S. enterica cells decreased by 0.5 or 2.4 log cycles at days 7 and 15 respectively, but still were detectable at day 30. E. coli O157 cells survived much better in salmorejo, decreasing only by 1.5 log cycles at day 30. Treatments at pressures of 400 MPa or higher reduced viable counts of L. monocytogenes and S. enterica to undetectable levels. HHP treatments significantly (P < 0.05) reduced E. coli counts by approximately 5.2 to 5.4 log cycles, but also yielded surviving cells that apparently were sublethally injured. Only samples treated at 600 MPA for 8 min were devoid of detectable E. coli cells during storage.

PRACTICAL APPLICATION

Salmorejo is a traditional, vitamin-rich food, usually produced on a small scale. HHP treatment at 600 MPa for 8 min can be an efficient nonthermal method for industrial-scale preparation of preservative-free salmorejo with improved safety against transmission of foodborne pathogens L. monocytogenes serotyes 4a and 4b, S. enterica serovar Enteritidis, and E. coli O157.

Rapid and sensitive detection of Listeria monocytogenes by cross-priming amplification of lmo0733 gene

Listeria monocytogenes is a food-borne pathogen that causes severe opportunistic infection in humans and animals. This study reports the development of single cross-priming amplification (S-CPA) and double CPA (D-CPA) assays targeting species-specific gene lmo0733 for identifying L. monocytogenes strains. The CPA assays were performed at a constant temperature 64 °C using seven specific primers and evaluated for specificity and sensitivity. The color change of positive amplification was directly observed by Loopamp^® Fluorescent Detection Reagent (FD), and the DNA products were visualized as a ladder-like banding pattern on 2.5% gel electrophoresis. Moreover, the positive reactions were also detected by real-time measurement of turbidity. 50 L. monocytogenes and 46 non-L. monocytogenes strains were used for the method verification, and the specificity was 100%. The limit of detection (LoD) of the S-CPA and D-CPA assays was 2.5 pg DNA per reaction and 10-fold more sensitive than PCR. A total of 60 pork samples were tested for L. monocytogenes using the S-CPA assay developed in the study, and the accuracy of the S-CPA and the culture-biotechnical method was 100% identical. The results suggested that the S-CPA assay was a rapid, sensitive, and valuable tool for detection of L. monocytogenes in food products.

Inactivation and sublethal injury kinetics of Staphylococcus aureus in broth at low temperature storage

Low temperatures are widely used to ensure food quality and safety. However, sublethally injured Staphylococcus aureus is an important microbiological safety concern in low temperature food. The objective of this study was to develop predictive inactivation kinetic models for the inactivation and sublethal injury of S. aureus in broth at different temperatures (4 to -18°C) and time points. S. aureus was diluted in tryptic soy broth plus 0.6% (wt/vol) yeast extract (TSBYE) to obtain approximately 10(8) CFU/ml and was stored separately at 4, -3, -11, and -18°C. After specific time points within 96 days, survival of S. aureus was determined on TSBYE and TSBYE agar plus 10% NaCl for enumeration of the total viable and noninjured cell numbers, respectively. Linear, Weibull, and modified Gompertz models were applied to determine survival curve regression. The combination of low temperature and time resulted in S. aureus inactivation, although the cells were able to survive in this sublethal state. Storage temperature was the critical parameter in survival of S. aureus. The modified Weibull model successfully described a second model of noninjured S. aureus cell survival at different low temperatures, whereas only the linear model was able to fit the total viable cells. The predictive model may be used to estimate the level of S. aureus contamination in food at low storage temperatures and times, and it provides new insight into the sublethally injured survival state of S. aureus in low temperature food.

Cyclic di-GMP-dependent signaling pathways in the pathogenic Firmicute Listeria monocytogenes

We characterized key components and major targets of the c-di-GMP signaling pathways in the foodborne pathogen Listeria monocytogenes, identified a new c-di-GMP-inducible exopolysaccharide responsible for motility inhibition, cell aggregation, and enhanced tolerance to disinfectants and desiccation, and provided first insights into the role of c-di-GMP signaling in listerial virulence. Genome-wide genetic and biochemical analyses of c-di-GMP signaling pathways revealed that L. monocytogenes has three GGDEF domain proteins, DgcA (Lmo1911), DgcB (Lmo1912) and DgcC (Lmo2174), that possess diguanylate cyclase activity, and three EAL domain proteins, PdeB (Lmo0131), PdeC (Lmo1914) and PdeD (Lmo0111), that possess c-di-GMP phosphodiesterase activity. Deletion of all phosphodiesterase genes (ΔpdeB/C/D) or expression of a heterologous diguanylate cyclase stimulated production of a previously unknown exopolysaccharide. The synthesis of this exopolysaccharide was attributed to the pssA-E (lmo0527-0531) gene cluster. The last gene of the cluster encodes the fourth listerial GGDEF domain protein, PssE, that functions as an I-site c-di-GMP receptor essential for exopolysaccharide synthesis. The c-di-GMP-inducible exopolysaccharide causes cell aggregation in minimal medium and impairs bacterial migration in semi-solid agar, however, it does not promote biofilm formation on abiotic surfaces. The exopolysaccharide also greatly enhances bacterial tolerance to commonly used disinfectants as well as desiccation, which may contribute to survival of L. monocytogenes on contaminated food products and in food-processing facilities. The exopolysaccharide and another, as yet unknown c-di-GMP-dependent target, drastically decrease listerial invasiveness in enterocytes in vitro, and lower pathogen load in the liver and gallbladder of mice infected via an oral route, which suggests that elevated c-di-GMP levels play an overall negative role in listerial virulence.

Listeria monocytogenes is ubiquitously present in the environment, highly adaptable and tolerant to various stresses. L. monocytogenes is also a foodborne pathogen associated with the largest foodborne outbreaks in recent US history. Signaling pathways involving the second messenger c-di-GMP play important roles in increased stress survival of proteobacteria and mycobacteria, yet roles of c-di-GMP signaling pathways in L. monocytogenes have remained unexplored. Here, we identified and systematically characterized functions of the proteins involved in c-di-GMP synthesis, degradation and sensing. We show that elevated c-di-GMP levels in L. monocytogenes result in synthesis of a previously unknown exopolysaccharide that promotes cell aggregation, inhibits motility in semi-solid media, and importantly, enhances bacterial tolerance to commonly used disinfectants as well as desiccation. These properties of the exopolysaccharide may increase listerial survival in food processing plants as well as on produce during transportation and storage. Elevated c-di-GMP levels also grossly diminish listerial invasiveness in enterocytes in vitro, and impair bacterial accumulation in selected mouse organs during oral infection.

Determination of Listeria monocytogenes Growth during Mushroom Production and Distribution

In the EU, food is considered safe with regard to Listeria monocytogenes if its numbers do not exceed 100 CFU/g throughout the shelf-life of the food. Therefore, it is important to determine if a food supports growth of L. monocytogenes. Challenge studies to determine the ability of a food to support growth of L. monocytogenes are essential as predictive modelling often overestimates the growth ability of L. monocytogenes. The aim of this study was to determine if growth of L. monocytogenes was supported during the production and distribution of mushrooms. A three-strain mixture of L. monocytogenes was inoculated onto three independent batches of whole mushrooms, sliced mushrooms, mushroom casing and mushroom substrate at a concentration of about 100–1000 CFU/g. The batches were incubated at potential abuse temperatures, as a worst case scenario, and at intervals during storage L. monocytogenes numbers, % moisture and pH were determined. The results showed that the sliced and whole mushrooms had the ability to support growth, while mushroom casing allowed survival but did not support growth. Mushroom substrate showed a rich background microflora that grew on Listeria selective media and this hindered enumeration of L. monocytogenes. In the case of this study, Combase predictions were not always accurate, indicating that challenge studies may be a necessary part of growth determination of L.monocytogenes.

Modeling the growth of Listeria monocytogenes on cut cantaloupe, honeydew and watermelon

A recent outbreak linked to whole cantaloupes underscores the importance of understanding growth kinetics of Listeria monocytogenes in cut melons at different temperatures. Whole cantaloupe, watermelon, and honeydew purchased from a local supermarket were cut into 10 ± 1 g cubes. A four-strain cocktail of L. monocytogenes from food related outbreaks was used to inoculate fruit, resulting in ~10(3) CFU/10 g. Samples were stored at 4, 10, 15, 20, or 25 °C and L. monocytogenes were enumerated at appropriate time intervals. The square root model was used to describe L. monocytogenes growth rate as a function of temperature. The model was compared to prior models for Salmonella and Escherichia coli O157:H7 growth on cut melon, as well as models for L. monocytogenes on cantaloupe and L. monocytogenes ComBase models. The current model predicts faster growth of L. monocytogenes vs. Salmonella and E. coli O157:H7 at temperatures below 20 °C, and agrees with estimates from ComBase Predictor, and a corrected published model for L. monocytogenes on cut cantaloupe. The model predicts ~4 log CFU increase following 15 days at 5 °C, and ∼1 log CFU increase following 6 days at 4 °C. The model can also be used in subsequent quantitative microbial risk assessments.