-Qualitative aspect of proteolytic activity of thermophilic lactobacilli using in Swiss cheeses. The average peptidic pattern was established from 253 French Emmental cheeses sampled in 50 factories. It contained 53 peptidic peaks, 4 of which were the most important in quantity. Two Lactobacillus helveticus strains and five Lactobacillus delbrueckii ssp. lactis strains were tested in Emmental experimental cheesemaking in association with three different propionic strains (21 cheeses). The Propionibacterium freudenreichii strains did not affect the peptidic pattern of ripened Emmental cheese. On the other hand, 13 peptidic peaks were significantly influenced by the thermophilic lactobacilli strain. The two species of thermophilic lactobacilli were discriminated by three peptidic peaks. Seven peptidic peaks were correlated with the ammonia amount in ripened Emmental cheese. No significant correlation was found between peptidic peaks and SN/TN and NPN/TN. At the same time, the peptidic pattern produced by the seven lactobacilli strains were studied in laboratory. The most specific peptidic peak of the French Emmental cheese were not found by using the Hammarsten casein as substrate. Another substrate, similar to hard cooked cheeses, lyophilized and sterilized by ionisation was tested. This substrate was obtained with microfiltered milk and Streptococcus thermophilus strains only. This substrate digested by thermophilic lactobacilli allowed to find the specific peptidic peaks of the French Emmental cheese. For example, the 14th peak obtained in laboratory was significantly different according to lactobacilli species, as it was observed in the Emmental cheese experiments. This peak distinguished the strains inside the Lb. delbrueckii ssp. lactis too.

Streptococcus thermophilus is a lactic acid bacterium widely used for manufacturing milk products. The protein composition of three strains of S. thermophilus grown in either M17 medium or milk was investigated. These conditions were chosen on the basis of the observed technological differences between these strains. Protein extracts prepared from cells harvested at the same pH (5.2 ± 0.1), were analyzed by two-dimensional gel electrophoresis. Gel comparison revealed qualitative and quantitative differences between the two growth media and between the strains. Some proteins with quantitative variation were partially identified. The most significant medium-related differences concerned the number of spots detected and the heat shock protein (GroEL, HSP70) levels which were higher in milk than in M17. On the contrary, the levels of enzymes (proton translocating ATPase, pyruvate kinase, tributyrin esterase, phospho-glycerate kinase, glyceraldehyde-3-phosphate dehydrogenase, fructose diphosphate aldolase, superoxide dismutase) and of a putative penicillin binding protein were lower in milk than in M17. Strain-related differences were accentuated in milk for heat shock proteins, tributryrine esterase and dinitrogenase reductase. Streptococcus thermophilus / milk / proteome / 2D-PAGE electrophoresis Résumé-Étude comparative de la composition protéique de trois souches de Streptococcus thermophilus cultivées dans le milieu M17 ou dans le lait. S. thermophilus est une bactérie lactique très utilisée pour la fabrication de produits laitiers. La composition protéique de trois souches de S. thermophilus cultivées dans le milieu M17 ou dans le lait a été étudiée. Ces conditions ont été choisies sur la base des différences technologiques observées entre les souches. Les extraits 645

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