The lag time of Listeria monocytogenes growing under suboptimal conditions (low nutrient concentrations, pH 6, and 6.5°C) was extended when the inoculum was severely stressed by starvation and the inoculum size was very small. Predictive microbiology should deal with bacterial stress and stochastic approaches to improve its value for the agro-food industry.In predictive microbiology, it is commonly assumed that inoculum size has no effect on subsequent microbial growth, and some studies (8, 9, 10) have confirmed this for growth of Listeria monocytogenes. However, Gay et al. (12) observed that inoculum size could have an effect on the duration of the lag phase of L. monocytogenes under particular conditions simulating soft cheese ripening. This observation could challenge the validity of predictive models because growth modelling is usually done with initial concentrations of bacteria higher than 10 3 CFU ⅐ ml Ϫ1 , even though foods are usually contaminated with lower numbers of cells.It has been shown with other bacteria, like Bacillus (19), Achromobacter delmarvae, Micrococcus luteus (see reference 16 for review), and Salmonella (26), that the duration of the lag phase depends inversely on the size of the inoculum. This phenomenon was observed only under a restricted range of conditions, for example, in poor but not in rich media, with starved cells (16), or with heat-injured cells (26). The aim of this work was to study the effect of inoculum size on the lag phase of L. monocytogenes growing in a poor medium, i.e., broth, containing only 1 g of Bacto Peptone (Difco Laboratories, Detroit, Mich.) per liter plus 0.85% sodium chloride (Prolabo, Paris, France) (TS), under suboptimal conditions: pH 6 and 6.5°C and with cells stressed by starvation.L. monocytogenes Scott A maintained on tryptone-soya agar (Oxoid, Unipath, Ltd., Basingstoke, Hampshire, England) at 4°C was subcultured onto tryptone-soya agar (Oxoid) plus 0.6% yeast extract (AES, Combourg, France) (TSYE) at 37°C for 24 h, and five colonies were transferred into TSYE broth and incubated at 30°C for 24 h. This culture was used to inoculate TSYE broth to prepare inocula at 30°C. The change in viable count of L. monocytogenes in TSYE broth at 30°C determined on nonselective TSYE agar and selective Palcam (Oxoid) agar is shown in Fig. 1. The percentage of injured cells was determined by differential counts on selective and nonselective media (24). Inocula were prepared at 30°C for 14, 160, and 840 h to obtain cells in late log phase, early stationary phase after a decrease in cell viability of 96.8% viable cells and 35% injured cells and late stationary phase with a loss of viability of 99.9% and 21% injured cells (Fig.

The growth potential of Listeria monocytogenes was evaluated at low temperature in sterilized milk and raw dairy products. Sterilized and raw milk were inoculated with different strains of L. monocytogenes in 2 physiological states and at various contamination levels. Raw cheese was naturally contaminated with Listeria spp. The results suggest that some biological factors influence the growth capacity of L. monocytogenes in dairy products. Significant strain effect was observed at low temperature whatever the growth medium. By contrast, no inoculum effect was observed in the 3 dairy products. In raw matrixes, growth of L. monocytogenes was influenced greatly by bacterial interactions and physiological state of inoculum cells.

Comment citer ce document : Buche, P., Dervin, C., Brouillaud-Delattre, A., . Combining fuzzy querying of imprecise data and predictive microbiology using category-based reasoning for prediction of the possible microbial spoilage in foods: application to Listeria monocytogenes.