Listeria monocytogenes Growth Kinetics in Milkshakes Made from Naturally and Artificially Contaminated Ice Cream

Assessing Listeria monocytogenes growth kinetics in rice pudding at different storage temperatures

Rice pudding is a popular artisanal dairy dessert highly consumed in the main rice-producing countries, including Egypt. This study aimed to evaluate and model the growth of Listeria monocytogenes in rice pudding dessert stored at different temperatures (4-25 °C) over its shelf-life. Lab-scale rice pudding samples were prepared following a traditional Egyptian recipe and inoculated with a three-strain cocktail of L. monocytogenes (ca. 3 × 102 cfu/g). Inoculated rice pudding samples (pH = 6.67 ± 0.06 and aw = 0.99 ± 0.002) were stored at different isothermal conditions (4, 8, 12, 18, and 25 °C) and microbiologically analysed for up to 30 days for pathogen quantification by plate count methodology. Global regression analysis was used to fit the Baranyi model coupled with the Ratkowsky model to growth data, relating L. monocytogenes concentrations (N, log cfu/g) with storage temperature (°C) and times (d). Model validation was performed using published independent data. L. monocytogenes growth potential increased by increasing storage temperature. The estimated Ratkowsky model parameters were b = 0.0819 ± 0.0017 log cfu/d·°C and Tmin = -3.28 ± 0.20 °C. The indices RMSE = 0.39 and R2adj = 0.97 indicated a good agreement between the experimental data and the model predictions. The estimated maximum growth rate (μmax) values ranged between 0.355 and 5.363 log cfu/d from 4 to 25 °C. The model was successfully validated using published L. monocytogenes Scott A and California strains growth data in rice pudding samples stored at 5, 12 and 22 °C, as evidenced by the assessed statistical indices. The predictive model developed and validated in this study will aid in decision-making regarding the microbiological safety of rice pudding dessert with respect to L. monocytogenes growth, considering a wide range of storage temperatures.

The Growth Curve of Microbial Cultures: A Model for a Visionary Reappraisal

A phenomenological model of planktonic microbial cultures, reported in previous papers, suggests that the whole growth progress seems planned by the microbial population since a pre-growth latency phase, during which the population level remains at its starting level. This model is in line with recent suggestions about the behavior of complex systems, as long as it allows for the gathering of the growth trends of a number of real batch cultures in a single master plot of reduced variables, in spite of their metabolic and physiological differences. One important issue of the model concerns the origin of the time scale for the microbes that can differ from that for the observer. The present paper reports some consequences of the model in view of its potential use in predictive microbiology and proposes an extension to the steady and decay phases of the culture evolution suggesting that, consistent with the assumptions about the growth phase, the decay occurs by a scan of the cell generation steps. This view leads to the conclusion that the steady phase between growth and decay trends actually corresponds to the loss of the oldest cell generations, which represents minor fractions of the microbial population. Such early decay is almost undetectable in a log scale, looking like a steady phase. To account for cases that show a broad maximum instead of an intermediate steady trend, a single continuous function, still related to the model, can describe the whole growth and decay trend of the microbial culture.

Effect of Time, Temperature, and Transport Media on the Recovery of Listeria monocytogenes from Environmental Swabs

ABSTRACT

Environmental monitoring for Listeria monocytogenes in food processing environments is key for ensuring the safety of ready-to-eat foods. For sampling, swabs are often hydrated with a wetting or transport medium that may contain neutralizers and other ingredients. After swabbing the environment, the swabs may then be transported or shipped cold to an off-site laboratory for testing, ideally within 48 h. Extended shipping times may subject the pathogen to increased temperatures in the presence of the wetting medium, organics, and other chemicals from the processing facility that could confound detection. This study evaluated growth and detection of L. monocytogenes on stainless steel exposed to either buffer or sodium hypochlorite before drying. Swabs were rehydrated with Butterfield's phosphate buffer, neutralizing buffer, Letheen broth, or Dey-Engley neutralizing broth before swabbing. Swabs were stored in the presence of no added food, cheese whey, or ice cream under both optimal (4°C) and suboptimal (15°C) temperatures for up to 72 h. Overall, there was no growth of L. monocytogenes at 4°C through 72 h of storage, although enrichment from these swabs was dependent on the presence and type of food matrix. Pathogen growth during storage at 15°C was more variable and depended on both the food matrix and transport media used, with Dey-Engley and Letheen broths allowing for the highest population increases. Overall, more enrichments resulting in L. monocytogenes detections were observed when using Letheen broth and neutralizing buffer than Dey-Engley broth, which resulted in fewer detections at 15°C. Logistic regression and Cochran-Mantel-Haenszel analyses determined that storage temperature, transport media, and food matrix all significantly affected detection of L. monocytogenes, whereas storage time did not have a clear effect on recovery from swabs.

HIGHLIGHTS

Dynamic kinetic analysis of growth of Listeria monocytogenes in pasteurized cow milk

The objective of this study was to develop a dynamic model for predicting the growth of Listeria monocytogenes in pasteurized cow milk under fluctuating temperature conditions during storage and temperature abuse. Six dynamic temperature profiles that simulated random fluctuation patterns were designed to change arbitrarily between 4 and 30°C. The growth data collected from 3 independent temperature profiles were used to determine the kinetic parameters and construct a growth model combining the primary and secondary models using a 1-step dynamic analysis method. The results showed that the estimated minimum growth temperature and maximum cell concentration were 0.6 ± 0.2°C and 7.8 ± 0.1 log cfu/mL (mean ± standard error), with the root mean square error (RMSE) only 0.3 log cfu/mL for model development. The model and the associated kinetic parameters were validated using the data collected under both dynamic and isothermal conditions, which were not used for model development, to verify the accuracy of prediction. The RMSE of prediction was approximately 0.3 log cfu/mL for fluctuating temperature profiles, and it was between 0.2 and 1.1 log cfu/mL under certain isothermal temperatures (2-30°C). The resulting model and kinetic parameters were further validated using 3 growth curves at 4, 7, and 10°C arbitrarily selected from ComBase (www.combase.cc). The RMSE of prediction was 0.8, 0.4, and 0.5 log cfu/mL, respectively, for these curves. The validation results indicated the predictive model was reasonably accurate, with relatively small RMSE. The model was then used to simulate the growth of L. monocytogenes under a variety of continuous and square-wave temperature profiles to demonstrate its potential application. The results of this study showed that the model developed in this study can be used to predict the growth of L. monocytogenes in contaminated milk during storage.

Attenuated Listeria monocytogenes as a Vaccine Vector for the Delivery of OMPW, the Outer Membrane Protein of Aeromonas hydrophila

Listeria monocytogenes (LM) is a gram-positive facultative intracellular pathogen that could stimulate host to produce inflammatory response, cell-mediated immunity, and humoral immunity. In this study, an attenuated live vector vaccine for Aeromonas hydrophila (AH) named EGDeABdd-dat-ompW was successfully constructed using an attenuated vector named EGDeABdd, in which dal, dat, actA, and inlB genes were deleted from wild-type LM-EGDe. To construct EGDeABdd-dat-ompW, a recombinant plasmid pERL3-dat-ompW obtained by inserting the dat gene from EGDe and outer membrane protein gene ompW from AH into pERL3 plasmid was transformed into EGDeABdd cell. The safety and immunogenicity of EGDeABdd-dat-ompW as an attenuated vector vaccine for delivery of OMPW were assessed through analyzing invasion to Caco-2 cells and mice, cytokine production of macrophagocyte and mouse splenocytes, and T-cell proliferation of mouse splenocytes. Serum titers against AH and the immunoprotective effect of the vaccine to mice were also measured after intravenous injection with vaccine for four times. The results showed that the live vector vaccine EGDeABdd-dat-ompW for AH exhibited high attenuation in invading Caco-2 cells and mice than did EGDe. Real-time PCR (RT-PCR) showed that cytokines (e.g., TNF-α, IL-6, and IL-1β from macrophages; and IL-6 and IFN-γ from mouse splenocytes) had significantly increased after immunization by EGDeABdd-dat-ompW. Meanwhile, the vaccine could induce the production of CD3⁺CD4⁺ and CD3⁺CD8⁺ T-cell proliferation of mice and generate effective immunoprotection against lethal challenge of 20 × LD₅₀ AH. All these results indicated that the attenuated EGDeABdd-dat could be used as a live vector for the delivery of the exogenous gene, not only possessing safety but also providing high immunogenicity. The successful application in the AH vaccine further showed that it could be used in other fields such as vaccines in cancer or infectious diseases.

An Assessment of Listeriosis Risk Associated with a Contaminated Production Lot of Frozen Vegetables Consumed under Alternative Consumer Handling Scenarios

Frozen foods do not support the growth of Listeria monocytogenes (LM) and should be handled appropriately for safety. However, consumer trends regarding preparation of some frozen foods may contribute to the risk of foodborne listeriosis, specifically when cooking instructions are not followed and frozen products are instead added directly to smoothies or salads. A quantitative microbial risk assessment model FFLLoRA (Frozen Food Listeria Lot Risk Assessment) was developed to assess the lot-level listeriosis risk due to LM contamination in frozen vegetables consumed as a ready-to-eat food. The model was designed to estimate listeriosis risk per serving and the number of illnesses per production lot of frozen vegetables contaminated with LM, considering individual facility factors such as lot size, prevalence of LM contamination, and consumer handling prior to consumption. A production lot of 1 million packages with 10 servings each was assumed. When at least half of the servings were cooked prior to consumption, the median risk of invasive listeriosis per serving in both the general and susceptible population was <1.0 × 10^-16 with the median (5th, 95th percentiles) predicted number of illnesses per lot as 0 (0, 0) and 0 (0, 1) under the exponential and Weibull-gamma dose-response functions, respectively. In scenarios in which all servings are consumed as ready-to-eat, the median predicted risk per serving was 1.8 × 10^-13 and 7.8 × 10^-12 in the general and susceptible populations, respectively. The median (5th, 95th percentile) number of illnesses was 0 (0, 0) and 0 (0, 6) for the exponential and Weibull-Gamma models, respectively. Classification tree analysis highlighted initial concentration of LM in the lot, temperature at which the product is thawed, and whether a serving is cooked as main predictors for illness from a lot. Overall, the FFLLoRA provides frozen food manufacturers with a tool to assess LM contamination and consumer behavior when managing rare and/or minimal contamination events in frozen foods.

Review of Electrochemical DNA Biosensors for Detecting Food Borne Pathogens

The vital importance of rapid and accurate detection of food borne pathogens has driven the development of biosensor to prevent food borne illness outbreaks. Electrochemical DNA biosensors offer such merits as rapid response, high sensitivity, low cost, and ease of use. This review covers the following three aspects: food borne pathogens and conventional detection methods, the design and fabrication of electrochemical DNA biosensors and several techniques for improving sensitivity of biosensors. We highlight the main bioreceptors and immobilizing methods on sensing interface, electrochemical techniques, electrochemical indicators, nanotechnology, and nucleic acid-based amplification. Finally, in view of the existing shortcomings of electrochemical DNA biosensors in the field of food borne pathogen detection, we also predict and prospect future research focuses from the following five aspects: specific bioreceptors (improving specificity), nanomaterials (enhancing sensitivity), microfluidic chip technology (realizing automate operation), paper-based biosensors (reducing detection cost), and smartphones or other mobile devices (simplifying signal reading devices).

Short communication: Long-term -20°C survival of Listeria monocytogenes in artificially and process-contaminated ice cream involved in an outbreak of listeriosis

Listeria monocytogenes was linked to an outbreak of foodborne illness associated with in-process contaminated ice cream in the United States from 2010 to 2015 that sickened 10 individuals and led to 3 deaths. Ice cream obtained from the outbreak was used in this study to examine the population dynamics of L. monocytogenes as in-process contaminants compared with artificially inoculated cells. Because challenge studies of food products generally use artificial contamination, it is necessary to understand the differences in survival, if any, between these 2 types of contaminants. We hypothesized that laboratory-grown cultures of the pathogen that were not exposed to the environmental stresses of the manufacturing facility would show different population dynamics in an ice cream challenge study compared with in-process contaminants. In this study, half of the outbreak-associated ice cream samples were artificially inoculated with a 10 cfu/g cocktail of L. monocytogenes; the other half contained only the in-process contaminants. All samples were stored at -20°C and tested for pathogen levels (n = 10 for each contaminant type at each time point) by the most probable number method at 3-mo intervals for 36 mo. Generally, population levels between the 2 contamination states in the ice cream were not significantly different and L. monocytogenes survived for at least 36 mo, regardless of contamination state. Overall, our results suggest that the use of L. monocytogenes as an artificial contaminant in challenge studies and risk assessment of ice cream during frozen storage give results similar to those shown by in-process contaminants.

New Term to Quantify the Effect of Temperature on pH min -Values Used in Cardinal Parameter Growth Models for Listeria monocytogenes

The aim of this study was to quantify the influence of temperature on pH min -values of Listeria monocytogenes as used in cardinal parameter growth models and thereby improve the prediction of growth for this pathogen in food with low pH. Experimental data for L. monocytogenes growth in broth at different pH-values and at different constant temperatures were generated and used to determined pH min -values. Additionally, pH min -values for L. monocytogenes available from literature were collected. A new pH min -function was developed to describe the effect of temperatures on pH min -values obtained experimentally and from literature data. A growth and growth boundary model was developed by substituting the constant pH min -value present in the Mejlholm and Dalgaard (2009) model (J. Food. Prot. 72, 2132-2143) by the new pH min -function. To obtain data for low pH food, challenge tests were performed with L. monocytogenes in commercial and laboratory-produced chemically acidified cheese including glucono-delta-lactone (GDL) and in commercial cream cheese. Furthermore, literature data for growth of L. monocytogenes in products with or without GDL were collected. Evaluation of the new and expanded model by comparison of observed and predicted μ max -values resulted in a bias factor of 1.01 and an accuracy factor of 1.48 for a total of 1,129 growth responses from challenge tests and literature data. Growth and no-growth responses of L. monocytogenes in seafood, meat, non-fermented dairy products, and fermented cream cheese were 90.3% correctly predicted with incorrect predictions being 5.3% fail-safe and 4.4% fail-dangerous. The new pH min -function markedly extended the range of applicability of the Mejlholm and Dalgaard (2009) model from pH 5.4 to pH 4.6 and therefore the model can now support product development, reformulation or risk assessment of food with low pH including chemically acidified cheese and cream cheese.