No abstract

Propionibacteria are naturally present in raw milk at low levels, but little is known regarding the influence of these wild-type strains on cheese quality. To evaluate the prevalence of wild strains of propionibacteria in Emmental cheese, three cheeses were manufactured from the same raw milk using three commercial Propionibacterium freudenreichii starters (A, B, and C). A total of 165 isolates from the raw milk and 479, 436, and 476 isolates from cheeses A, B, and C, respectively, were typed by insertion sequence-restriction fragment length polymorphism or by multiple-locus variable number of tandem repeat analysis in order to determine the composition of the propionibacterial flora in the raw milk, in the curd, and in the cheeses after 2, 4, 6, and 8 months of ripening. Nine starter strains and 17 wild P. freudenreichii strains isolated from the curd and cheese were characterized with regard to their specific activity of aspartase and for their ability to grow at low temperature (11°C). After 8 months of ripening, more than 80% of wild-type strains of P. freudenreichii were isolated from cheese A, whose starter strains had low aspartase activity. Although warm room storage (23°C) stimulated the growth of various wild-type strains in all cheeses, strains of starters B and C, which had high aspartase activity or fast growth at 11°C, remained dominant over the whole ripening period. In all cheeses, the growing wildtype strains exhibited high aspartase activity, indicating that this strain-specific property is a key factor in the control of propionic acid fermentation and in the prevention of late fermentation in Swiss-type cheeses.

-A general method for the detection and identification of specific strains of bacteria is described. The assay is based on the observation that insertion sequences (IS) in different strains of bacteria occur at diverse loci on the bacterial genome. Exclusive PCR primers can be selected for a particular strain where one of the primers is specific for a particular IS element and the other is specific for the adjacent DNA sequence in the genome. Only bacterial strains containing the IS element at the particular point on the genome will yield an amplicon of the expected size after PCR. We have illustrated this method by selecting primers to detect some lactobacilli strains that are used exclusively in the manufacture of Swiss Emmental cheese. Using this method we were able to differentiate cheeses manufactured in Switzerland from those made in other European countries.transposase / insertion sequence / strain-specific PCR / lactic acid bacteriaRésumé -Marqueurs génétiques naturellement présents chez les lactobacilles et leur utilisation pour vérifier l'authenticité du fromage Emmental suisse AOC. Une méthode générale pour la détection et l'identification spécifique de souches bactériennes est décrite. Elle est basée sur l'observation que les séquences d'insertion (IS) sont présentes à divers endroits sur le génome bactérien. Des amorces PCR spécifiques à une souche peuvent être conçues, l'une étant située sur un élément IS et l'autre sur la séquence ADN adjacente. Seules les souches avec l'élément IS à cet endroit particulier du génome donneront un produit d'amplification de la taille attendue après PCR. Cette méthode est illustrée ici par des paires d'amorces permettant de détecter spécifiquement des souches de lactobacilles utilisées exclusivement dans la fabrication de l'Emmental suisse. Ainsi, nous avons pu différencier les fromages fabriqués en Suisse des fromages provenant d'autres pays européens.