-Listeria monocytogenes is a foodborne pathogenic bacterium sometimes found in raw milk. Raw milk contains natural bacterial inhibitors such as the lactoperoxidase system (LPS) and specific microflora. Six strains of L. monocytogenes isolated from raw milk in 1995 and 1996 in Normandy (France) were tested. The aim of the first part of this work was to evaluate the inhibitory effect of LPS on L. monocytogenes in milk. Kept at 15 °C for 65 h, in static conditions, populations of L. monocytogenes in pasteurized milk increased by 2 to 3.8 log depending on the strain. In raw milk, in the same conditions, populations increased by 0.8 to 2.3 log. Adding thiocyanate and hydrogen peroxide to raw milk (supplemented raw milk, SRM) enhanced its inhibitory effect. In SRM, three strains were unable to grow and the populations of the other strains increased by 0.7 to 1.3 log. The inhibitory effect of the LPS in milk was clearly demonstrated. The inhibitory effect of raw milk on L. monocytogenes was due to LPS, probably combined with the microbiological composition of raw milk. The aim of the second part of this work was to evaluate the inhibitory effect of using raw milk for making Camembert cheese (RMC). The results show that the growth of L. monocytogenes was about twice as slow in RMC as in Camembert made from pasteurized milk (PMC). The average lag phase (Lag) was 15 d in PMC and 34 d in RMC. Statistical analysis showed that the inhibitory effect of RMC on the growth of L. monocytogenes was mainly related to the microbiological composition of the raw milk, in terms of thermophilic Lactobacillus and yeast. Although our results did not clearly demonstrate an inhibitory effect of chemical composition of raw milk, inhibition of L. monocytogenes in RMC is probably due to the interrelationship between microbiological and chemical factors. Listeria monocytogenes / inhibition / raw milk / lactoperoxidase systemRésumé -Facteurs ralentissant la croissance de Listeria monocytogenes dans le lait cru et dans les fromages à pâte molle au lait cru. Six souches de L. monocytogenes isolées de lait cru en 1995 et 1996 (Normandie) sont utilisées. L'effet du système lactopéroxydase (LPS), inhibiteur de croissance microbienne naturellement présent dans le lait cru, sur L. monocytogenes, est évalué. Dans le lait pasteurisé incubé 65 h à 15 °C, la population de L. monocytogenes, augmente de 2 à 3,8 log selon les souches. Dans le lait cru, la population augmente de 0,8 à 2,3 log. L'ajout de thiocyanate et de péroxyde d'hydrogène au lait cru (SRM) accroît son effet inhibiteur. Dans le SRM, trois souches ne se développent pas ; pour les autres, la population augmente de 0,7 à 1,3 log. L'effet inhibiteur du lait cru sur L. monocytogenes est lié à la présence de LPS, et probablement à la composition microbiologique du lait cru. La croissance de L. monocytogenes dans le Camembert est également étudiée. Les résultats montrent que L. monocytogenes se développe environ deux fois plus lentement dans le Camembert au lait cru (RMC) que dans celui a...

The influence of the bacterial concentration of an inoculum (10(1) or 10(3) cfu ml-1) of two strains of Listeria monocytogenes (Scott A: serotype 4b and V7: serotype 1) and one strain of L. innocua (Lin 11), and the time and temperature at which the inoculum was stored (cold storage: 4 degrees C for 4 weeks, or without cold storage: -20 degrees C before immediate transfer), and the temperature at which cells were pre-incubated (30 degrees C and 14 degrees C) on subsequent growth in Richard's broth at 14 degrees C was investigated. Richard's broth at a pH 5.9 was used to simulate potential growth in soft cheese (camembert type) and an incubation temperature of 14 degrees C was used to simulate storage-temperature ripening of cheese. Enumeration of the number of viable cells was by plate count method, except where viable cell numbers were less that 10(3) cfu ml-1, when the MPN (Most Probable Number) technique was used. With cold storage and an inoculum of 10(3) cfu ml-1 (high bacterial concentration) the pre-incubation temperatures (30 degrees C and 14 degrees C) did not significantly influence the subsequent growth curve: there was no significant lag (less that 21 h) and cell numbers peaked in about 8.5 d. However, with cold storage and an inoculum of 10(1) cfu ml-1 (low bacterial concentration) and a pre-incubation temperature of 30 degrees C a significant shift in the growth curve was observed over that pre-incubated at f14 degrees C, with the appearance of a lag of about 7.7 d. At a pre-incubation temperature of 14 degrees C with the low inoculum concentration, there was a measurable lag of about 1 d. Without cold storage and a pre-incubation temperature of 30 degrees C, there was a lag time of 2.3 d. Storage conditions, pre-incubation temperature and inoculum concentration therefore appear to influence the subsequent growth curve. Importantly, however, the growth curves for cultures from inocula, pre-incubated at either 30 degrees C or 14 degrees C, appeared to involve two distinct values of the exponential growth rate (k): the initial portion of the growth curve described by a low value of k and the subsequent portion by a consistently and significantly greater value. The appearance of two distinct growth phases was reproduced in further data determined for all the studied strains of the microorganism. Further study to explain these unexpected and reproducible findings is being conducted.

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The effect of preheating on the survival of L. monocytogenes in Richard's broth, which mimics the composition of Camembert cheese composition, was examined. Experiments were carried out to reproduce contamination of cheese with environmental heat-stressed cells of L. monocytogenes surviving hot-cleaning procedures. Cells in mid-log phase were heated for 30 min at 56 degrees C before being inoculated into Richard's broth. The pHs and temperatures of Richard's broth were chosen to recreate the conditions of curd dripping (pH 5, 25 degrees C), of the beginning of cheese ripening (pH 5, 12 degrees C), and of the beginning (pH 5, 4 degrees C) and the end (pH 7, 4 degrees C) of cheese storage. Immediately after heat treatment, the viability loss was especially high for strain 306715, which exhibited only 0.6% +/- 0.2% survival, compared with 22% +/- 8.7% for strain EGD. The percentages of the surviving heated cells that were injured were 93% +/- 8% for strain 306715 and 98% +/- 3% for strain EGD. The destruction of the surviving L. monocytogenes cells was accelerated when they encountered the pH and temperature conditions of Camembert cheese during manufacturing, ripening, and cold storage (pH 5 at 25, 12, and 4 degrees C, respectively). The multiplication of the surviving heated cells was retarded under favorable growth conditions similar to those of storage by the distributor and the consumer (pH 7 at 4 and 12 degrees C, respectively).