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Hereby, we showed that the experimental and biological variability in culture preparation had little effect on the stochastic outcome of: (i) growth, (ii) relative transcription of stress- ( gad2, sigB ) and virulence- ( prfA ) associated genes and (iii) subsequent acid resistance of Listeria monocytogenes , across growth/ no growth boundaries regarding combinations of pH (4.8-7.2) (HCl) and NaCl (0-8% w/v) at 7 o C. Variability of bacterial response, as described by the coefficient of variation (CV) and root mean square error (RMSE) was affected mainly by the previously pH and NaCl conditions that the pathogen had experienced. High biological variability regarding growth potential of the pathogen, was observed in conditions across growth boundaries (at 7 o C), such as pH 5.5-6.4 and NaCl 2-8% w/v, as manifested by the highly ranged growth parameters (CV areas 18.3% - 49%, RMSE areas 8.7-13.9, RMSE lag time 9.3-31.2), while acid resistance (pH 2.0, HCl, 37 °C) was highly variable when pathogen habituated pH (5.0-5.2) and NaCl (2% w/v) resulting in average D1 (3-6 min) and D2 (14-16 min) of high variability (CV D1 28-35%; CV D2 35-60%). Moreover at the same conditions the highest upregulation of gad2 was observed with high biological variability of RMSE gad2 2.8, while relative transcription levels of pfrA ranged from 0.60 to 4.22, indicating the potential risk derived from the stochastic bacterial response (up- or down- regulation) regarding induction of virulence mechanisms in growth boundary conditions. [ABSTRACT FROM AUTHOR]

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 (aw) levels with a mix of two strains of L. monocytogenes. A factorial design of five pH values (5.6 to 6.5), tour aw 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 aw 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 aw of 0.99 for low contamination levels and pH 6.5 and aw, 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. [ABSTRACT FROM AUTHOR]

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 109 CFU ml-1, as stipulated by the LOOC 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. [ABSTRACT FROM AUTHOR]

To describe the growth limits of Lisieria 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°C) and water activity (from 0.88 to 0.98). Two inoculum concentrations were used (10 and 104 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°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 (µmax), lag phase duration (LPD), and maximum cell concentration (Nmax) derived from the primary model were modeled using the square root function (µmax 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. [ABSTRACT FROM AUTHOR]

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 (aw) 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 (10² 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 (aw 0.995), or four aw 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 10² CFU/ml) to each of the aforementioned growth-permitting pH and aw 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 aw 0.90 (12.5% NaCl) for 1, 5, and 10 days to all pH and aw levels permitting growth. Reducing aw 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 aw (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. [ABSTRACT FROM AUTHOR]