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Peracetic acid (PAA), a strong oxidizing agent, has been widely used as a disinfectant in food processing settings as it does not produce harmful chlorinated by-products. In the present study, the transcriptional response of Listeria monocytogenes to a sub-lethal concentration of PAA (2.5 ppm) was assessed using RNA-sequencing (RNA-seq). Our analysis revealed 12 differentially expressed protein-coding genes, of which nine were upregulated (ohrR, ohrA, rpsN, lmo0637, lmo1973, fur, lmo2492, zurM, and lmo1007), and three were down-regulated (argG, lmo0604 and lmo2156) in PAA-treated samples compared to the control samples. A non-coding small RNA gene (rli32) was also found to be down-regulated. In detail, the organic peroxide toxicity protection (OhrA-OhrR) system, the metal homeostasis genes fur and zurM, the SbrE-regulated lmo0636-lmo0637 operon and a carbohydrate phosphotransferase system (PTS) operon component were induced under exposure of L. monocytogenes to PAA. Hence, this study identified key elements involved in the primary response of L. monocytogenes to oxidative stress caused by PAA, including the expression of the peroxide detoxification system and fine-tuning the levels of redox-active metals in the cell. The investigation of the molecular mechanism of PAA response in L. monocytogenes is of utmost importance for the food industry, as residual PAA can lead to stress tolerance in pathogens., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Salmonella enterica is capable of entering the interior of leafy greens and establishing in the apoplastic area, a phenomenon known as internalization. The ability of internalized bacteria to evade common disinfection practices poses a well-established risk. Our aim was to study the effect of: i) inoculum size and ii) prior adaptation of Salmonella to sublethal stresses, on the internalization of the pathogen in four leafy vegetables. Spinach, lettuce, arugula and chicory were inoculated, by immersion for 2 min at room temperature with: i) Salmonella Enteritidis at 3.0, 4.0, 5.0, 6.0, 7.0 log CFU/mL and ii) non-adapted or adapted S. Enteritidis to acid (in TSB with 1% glucose, incubated for 24 h at 37 °C), cold (in TSB for 7 days at 4 °C), starvation (0.85% NaCl of pH 6.6, 48 h at 37 °C) or desiccation (1.5 h at 42 °C, 4 days at 21 °C) stress at appx 3.5 log CFU/mL). Inoculated leafy greens were subsequently stored at 5 °C and 20 °C for 2 h and 48 h (n = 2 × 2). Population of internalized Salmonella, after surface decontamination with 1% w/v AgNO 3 , was assessed on selective media. Even the lowest initial bacterial inoculum was adequate for internalization of Salmonella to occur in leafy vegetables. Non-adapted Salmonella inoculum of 7.0 (maximum) and 3.0 log CFU/mL (lowest inoculation level tested) after short storage (2 h) resulted in 3.7-4.3 and 1.3-1.5 log CFU/g internalized bacterial population, respectively. Colonization (including both attachment and internalization processes), as well as internalization process, were positively correlated to initial inoculum level. These processes reached a different plateau beyond which, no further increase in internalization was observed. Adaptation of the pathogen to mild stresses enhanced internalization (P < 0.05), with desiccation- and acid-adapted Salmonella demonstrating the highest internalization capacity, regardless of the vegetable and storage temperature. These findings could contribute to further elucidation of colonization capacity of Salmonella in leafy vegetables and assist in selecting the proper conditions that contribute to the prevention of fresh produce contamination with Salmonella., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Due to climate change, with contaminated and less fertile soils, and intense weather phenomena, a turn towards hydroponic vegetable production has been made. Hydroponic cultivation of vegetables is considered to be a clean, safe and environmentally friendly growing technique; however, incidence of microbial contamination i.e. foodborne pathogens, might occur, endangering human health. The aim of this study was to investigate the effects of different plant growth stages, pH (values 5, 6, 7, 8) and bacterial inoculum levels (3 and 6 log cfu/mL) on hydroponically cultivated lettuce spiked with Salmonella Enteritidis. The results revealed that the pH and inoculum levels affected the internalization and survival of the pathogen in the hydroponic environment and plant tissue. Younger plants were found to be more susceptible to pathogen internalization compared to older ones. Under the current growing conditions (hydroponics, pH and inoculum levels), no leaf internalization was observed at all lettuce growth stages, despite the bacterium presence in the hydroponic solution. Noticeably, bacteria load at the nutrient solution was lower in low pH levels. These results showed that bacterium presence initiates plant response as indicated by the increased phenols, antioxidants and damage index markers (H 2 O 2 , MDA) in order for the plant to resist contamination by the invader. Nutrient solution management can result in Taylor-made recipes for plant growth and possible controlling the survival and growth of S. Enteritidis by pH levels., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Fresh vegetables are important components of an everyday balanced diet making ready to-eat-salads (RTE) a commodity widely consumed. However, in the past few years these products have been linked with outbreaks of salmonellosis and listeriosis; thus the continuous investigation of their safety is an essential requirement. A total of 216 samples of ready-to-eat salads from the Cypriot market were analysed to determine the microbiological quality and safety, along with physicochemical attributes of the salads and identify possible correlations between them. The samples were randomly collected from four retail outlets and correspond to five different salad producing companies. Furthermore, the effects of season, salad producer and type of salad and/or their interactions with the tested parameters were investigated. The results revealed that the higher microbial load among seasons was observed in samples collected during spring. Escherichia coli was found in 11.57% of samples and 2.62% of isolates were found to be able to produce extended spectrum β-lactamase (ESBL). All samples were found negative for Salmonella enterica, whereas Listeria monocytogenes was present in 3.70% of samples. Higher levels of spoilage bacteria (lactic acid bacteria and Pseudomonas spp.) were detected during winter and spring. Additionally, the %CO 2 production was affected by the type of salad, while the interaction between producer and type of salad, affected total phenolic content and antioxidant activity of samples. A positive correlation of phenols and antioxidants with the presence of Staphylococcus spp., Pseudomonas spp., E. coli and Bacillus cereus was observed, suggesting that excessive handling increases microbial load and plant stress., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

The objective of this study was to correlate the relative transcription of Listeria monocytogenes (strains C5, 6179) stress- (gad2, sigB) and virulence- (prfA) associated genes following habituation at twenty-five pH (4.8, 5.0, 5.2, 5.5, 6.4) and NaCl (2, 4, 6, 8, 10% w/v) combinations at 7 °C, with the survival against subsequent exposure to severe acid stress (pH 2.0 at 37 °C). Our findings pointed out the inter-strain variation governing growth inhibiting conditions (pH ≤5.0 and NaCl ≥6%), where C5 was less affected (a reduction of 2.0-3.0 log CFU/mL) than 6179 which was reduced by 4.0-6.0 log CFU/mL at the end of storage. Nevertheless, the higher the habituation at the growth permitting (pH ≥5.5; NaCl ≤4% w/v) or growth inhibiting conditions, the higher the acquired acid resistance or sensitization, respectively. At day 2, gad2 increased relative transcriptional levels are more related to elevated acid resistance, while at day 6 both gad2 transcriptional levels and upregulation of sigB were correlated to low log reductions and high DpH:2.0-values against severe acid stress. Regarding virulence, the increased transcriptional levels of prfA at day 2 were correlated to adverse pH and NaCl combinations, while prolonged stay in suboptimal conditions as well as exposure to severe acid stress resulted in general activation of the virulence regulator. Such data could definitely contribute in designing safe intervention strategies and additionally integrate -omics aspects in quantitative microbial risk assessment., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

In this study, mathematical models were developed to predict the growth probability and kinetic behavior of Listeria monocytogenes on fresh pork skin during storage at different temperatures. A 10-strain mixture of L. monocytogenes was inoculated on fresh pork skin (3 by 5 cm) at 4 log CFU/cm(2). The inoculated samples were stored aerobically at 4, 7, and 10 °C for 240 h, at 15 and 20 °C for 96 h, and at 25 and 30 °C for 12 h. The Baranyi model was fitted to L. monocytogenes growth data on PALCAM agar to calculate the maximum specific growth rate, lag-phase duration, the lower asymptote, and the upper asymptote. The kinetic parameters were then further analyzed as a function of storage temperature. The model simulated growth of L. monocytogenes under constant and changing temperatures, and the performances of the models were evaluated by the root mean square error and bias factor (Bf). Of the 49 combinations (temperature × sampling time), the combinations with significant growth (P < 0.05) of L. monocytogenes were assigned a value of 1, and the combinations with nonsignificant growth (P > 0.05) were given a value of 0. These data were analyzed by logistic regression to develop a model predicting the probabilities of L. monocytogenes growth. At 4 to 10 °C, obvious L. monocytogenes growth was observable after 24 h of storage; but, at other temperatures, the pathogen had obvious growth after 12 h of storage. Because the root mean square error value (0.184) and Bf (1.01) were close to 0 and 1, respectively, the performance of the developed model was acceptable, and the probabilistic model also showed good performance. These results indicate that the developed model should be useful in predicting kinetic behavior and calculating growth probabilities of L. monocytogenes as a function of temperature and time.

Listeria monocytogenes can proliferate at the beginning of cheesemaking as the conditions favor growth. The objective of this study was to establish the growth limits of L. monocytogenes in a cheese matrix, in case of potential contamination of the milk prior to cheese manufacture. A semisoft laboratory scale model cheese system was made at different initial pH and water activity (a(w)) levels with a mix of two strains of L. monocytogenes. A factorial design of five pH values (5.6 to 6.5), four a(w) values (0.938 to 0.96), and two L. monocytogenes inoculation levels (1 to 20 CFU/ml and 500 to 1,000 CFU/ml) was carried out. Each combination was evaluated in six independent replicates. In order to determine if there was a dominant strain, isolated colonies from the cheeses were analyzed by pulsed-field gel electrophoresis. The data relating to growth initiation were fitted to a logistic regression model. The a(w) of milk influenced the probability of growth initiation of L. monocytogenes at both low and high contamination levels. The pH, at the concentrations tested, had a lower effect on the probability of growth initiation. At pH 6.5 and a(w) of 0.99 for low contamination levels and pH 6.5 and a(w) of 0.97 for high contamination levels, increases in population of up to 4 and 2 log were observed at low and high contamination levels, respectively. This shows that if conditions are favorable for growth initiation at the early stages of the cheesemaking process, contamination of milk, even with low numbers, could lead to L. monocytogenes populations that exceed the European Union's microbiological limit of 100 CFU/g of cheese.

The aim of the present study was to develop a predictive model to quantify the spoilage of yoghurt with fruits. Product samples were stored at various temperatures (5-20 °C). Samples were subjected to microbiological (total viable counts, lactic acid bacteria-LAB, yeasts and moulds) and physico-chemical analysis (pH, titratable acidity and sugars). LAB was the dominant micro-flora. Yeasts population increased at all temperatures but a delay was observed during the first days of storage. Titratable acidity and pH remained almost constant at low temperatures (5 and 10 °C). However, at higher temperatures (>10 °C), an increase in titratable acidity and reduction in pH was observed. Sugar concentration (fructose, lactose and glucose) decreased during storage. A mathematical model was developed for shelf-life determination of the product. It was successfully validated at a temperature (17 °C) not used during model development. The results showed that shelf-life of this product could not be established based only on microbiological data and use of other parameters such as sensory or/and physico-chemical analysis is required. Shelf-life determination by spoilage tests is time-consuming and the need for new rapid techniques has been raised. The developed model could help dairy industries to establish shelf-life predictions on yoghurt with fruits stored under constant temperature conditions., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

The effect of acid and osmotic shifts on the growth of Listeria monocytogenes was evaluated at 10°C. Two types of shifts were tested: (i) within the range of pH and water activity (a(w)) levels that allow growth of L. monocytogenes and (ii) after habituation at no-growth conditions back to growth-permitting conditions. A L. monocytogenes cheese isolate, with high survival capacity during cheesemaking, was inoculated (10(2) CFU/ml) in tryptic soy broth supplemented with 0.6% yeast extract at six pH levels (5.1 to 7.2; adjusted with lactic acid) and 0.5% NaCl (a(w) 0.995), or four a(w) levels (0.995 to 0.93, adjusted with 0.5 to 10.5% NaCl) at pH 7.2 and grown to early stationary phase. L. monocytogenes was then shifted (at 10(2) CFU/ml) to each of the aforementioned growth-permitting pH and a(w) levels and incubated at 10°C. Shifts from no-growth to growth-permitting conditions were carried out by transferring L. monocytogenes habituated at pH 4.9 or a(w) 0.90 (12.5% NaCl) for 1, 5, and 10 days to all pH and a(w) levels permitting growth. Reducing a(w) or pH at different levels in the range of 0.995 to 0.93 and 7.2 to 5.1, respectively, decreased the maximum specific growth rate of L. monocytogenes. The lag time of the organism increased with all osmotic downshifts, as well as by the reduction of pH to 5.1. Conversely, any type of shift within pH 5.5 to 7.2 did not markedly affect the lag times of L. monocytogenes. The longer the cells were incubated at no-growth a(w) (0.90), the faster they initiated growth subsequently, suggesting adaptation to osmotic stress. Conversely, extended habituation at pH 4.9 had the opposite effect on subsequent growth of L. monocytogenes, possibly due to cell injury. These results suggest that there is an adaptation or injury rate induced at conditions inhibiting the growth of the pathogen. Thus, quantifying adaptation phenomena under growth-limiting environments, such as in fermented dairy and meat products or products preserved in brine, is essential for reliable growth simulations of L monocytogenes during transportation and storage of foods.

A cooked, cured meat product is a perishable product spoiled mainly by lactic acid bacteria (LAB). LAB cause discoloration, slime formation, off-odors and off-flavors as the result of their metabolic activity producing various products. These microbial products in conjunction with the microbial population could be used to assess the degree of spoilage of this type of product. The spoilage evaluation was achieved by following a multivariate approach. Cluster analysis, principal component analysis and partial least square regression were employed to associate spoilage with microbiological and physicochemical parameters. The developed model was capable of giving accurate predictions of spoilage describing the spoilage associations. The study might contribute to the improvement of quality assurance systems of meat enterprises.

The physicochemical, microbiological, and organoleptic profile of different commercial table olive products from retail outlets was studied. Average pH values were 4.00, 3.96, and 4.31 for Spanish-style green, naturally black, and dry-salted olives, respectively, while salt content was 6.21, 7.34, and 8.00% for the same commercial products. Mean values for titratable acidity were 0.53 and 0.63% (wt/vol) for green and naturally black olives. In general, mean values for pH, titratable acidity, and salt content were in accordance with the requirements established by the International Olive Oil Council (IOOC) for the trade of table olives, although considerable variation was observed within individual olive samples. Salt content of dry-salted olives did not meet the minimum limit of 10% established by the IOOC. The dominant microbiota consisted of lactic acid bacteria and yeasts. Their population was less than 10(9) CFU ml(-1), as stipulated by the IOOC standard for fermented olives held in bulk in a covering liquid. These microorganisms come from the natural microbiota found in spontaneous fermentations and impose no risk to human health. No enterobacteria, pseudomonads, Bacillus cereus, or Clostridium perfringens were detected in any of the samples given the physicochemical characteristics found. The organoleptic profile varied greatly according to processing style and commercial preparation. Green olives had more uniform sensory characteristics than naturally black and dry-salted olives. The most important attributes that influenced the judgment of the panelists were salt content and crispness of the olives.

To describe the growth limits of Listeria monocytogenes NCTC10527 in a sliced vacuum-packaged cooked cured meat product, the binary logistic regression model was used to develop an equation to determine the probability of growth or no growth of L. monocytogenes as a function of temperature (from 0 to 10 degrees C) and water activity (from 0.88 to 0.98). Two inoculum concentrations were used (10 and 10(4) CFU g(-1)), and the growth limits for the two inocula were different. The kinetic behavior of L. monocytogenes as a function of temperature (4, 8, 12, and 16 degrees C) on the same meat product at the lower concentration (10 CFU g(-1)) was also studied. The Baranyi model appeared to fit the overall experimental data better than did the modified Gompertz and the modified logistic models. Maximum specific growth rate (micromax), lag phase duration (LPD), and maximum cell concentration (Nmax) derived from the primary model were modeled using the square root function (micromax and LPD) and a second order polynomial (Nmax) (secondary models). The selection of the best model (primary or secondary) was based on some statistical indices (the root mean square error of residuals of the model, the regression coefficient, the F test, the goodness of fit, and the bias and accuracy factor). The developed kinetic behavior model was validated under constant and dynamic temperature storage conditions. This prediction of L. monocytogenes growth provides useful information for improving meat safety and can be used for in-depth inspection of quality assurance systems in the meat industry.

A dynamic model was developed to describe the microbial interaction and the death of Escherichia coli O157:H7 during the fermentation of green table olives with two different starter cultures supplemented with different amounts of glucose and sucrose. The model consists of six differential equations including substrate (glucose or sucrose) consumption and product inhibition (protons and protonated lactic acid). Experimental data from a multifactorial experiment were used to fit the model, and the model was verified with independent data. Yeasts, which were the only competitors of starters, managed to reach levels equal to those of starters by the end of fermentation. The decrease in the level of Escherichia coli O157:H7 was proportional to the initial amounts of glucose and sucrose added. However, in the majority of cases, the death of the pathogen occurred in three successive phases, with a short mediate survival period. The production of lactic acid during fermentation seems to be the main factor governing the behavior of this pathogen under such stress conditions. Therefore, the death of E. coli was modeled with a differential equation that included the effects of pH, protonated lactic acid, and the protective effect of the substrate.

The effect of gamma irradiation on the natural microflora of whole salted vacuum-packaged trout at 4 and 10 degrees C was studied. In addition, the effectiveness of gamma irradiation in controlling Listeria monocytogenes inoculated into trout was investigated. Irradiation at doses of 0.5 and 2 kGy affected populations of bacteria, namely, Pseudomonas spp., Brochothrix thermosfacta, lactic acid bacteria, H2S-producing bacteria typical of Shewanella putrefaciens, and Enterobacteriaceae, at both 4 and 10 degrees C. This effect was more pronounced at the higher dose (2 kGy) and the lower temperature (4 degrees C). Pseudomonads, H2S-producing bacteria typical of S. putrefaciens, and Enterobacteriaceae showed higher sensitivity to gamma irradiation than did the rest of the microbial species. Sensory evaluation did not show a good correlation with bacterial populations. On the basis of sensory odor scores, a shelf life of 28 days (2 kGy, 4 degrees C) was obtained for salted vacuum-packaged freshwater trout, compared with a shelf life of 7 days for the unirradiated sample. Under the same conditions, the growth of L. monocytogenes inoculated into the samples was suppressed by 2 log cycles after irradiation (2 kGy) and storage for up to 18 days at 4 degrees C.