Three galactose-positive (Gal+) mutants were isolated after treating Streptococcus thermophilus strain CNRZ 302 with N-methyl-N′-nitro-N-nitrosoguanidine; other physiological and biochemical characteristics were retained. In contrast with the wild type's inability to utilize galactose, the A5 mutant metabolized 70% of the galactose resulting from the hydrolysis of lactose. Phosphoenolpyruvate-phosphotransferase activity could not be detected, indicating that this system was not involved in galactose transport. Galactokinase activities were higher in induced galactose-fermenting strains than in the galactose-negative strain. The mutation seemed to affect a galactokinase regulation gene. Key words: galactokinase, galactose, hydrolysis of lactose, mutants, Streptococcus thermophilus.
La fermentation du lactose du lait par les bactéries lactiques, essentiellement les genres Streptococcus et Lactobacillus, est primordiale en Industrie Laitière. Depuis une dizaine d'années, les systèmes de transport et d'hydrolyse des glucides par ces microorganismes ont été étudiés. Relativement aux autres bactéries du même genre, il existe peu de données sur le métabolisme glucidique chez Streptococcus thermophilus.La présente étude a porté sur l'effet de la source d'énergie sur la croissance et la prise de l'acide glutamique chez différentes espèces de streptocoques (35 souches appartenant à 14 espèces). Nous avons pu constater l'originalité de S. thermophilus : le lactose est un meilleur substrat que le glucose aussi bien pour le transport actif que pour la croissance de la bactérie. Tous les autres streptocoques testés sauf peut-être S. salivarius préfèrent le glucose ou donnent des résultats similaires avec les deux glucides. L'adaptation préalable des streptocoques du groupe N au lactose les rend plus aptes à fermenter ce diholoside.Mots clés: Streptococcus -Prise de l'acide glutamique -Lactose -Glucose -Adaptation. Summary Influence of energy source on the growth and glutamic acid uptake by Streptococcus genusMilk lactose fermentation by lactic acid bacteria, especially Streptococcus and Lactobacillus genera is important in the Dairy Industry. For about ten years, the systems of sugar transport and subsequent hydrolysis by these microorganisms have been studied. With respect to the other bacteria of the same genus, very few data on carbohydrate metabolism in Streptococcus thermophilus are available whereas a great amount of research attention has been given to this fermentation by the group N streptococci.The present study examines the influence of the energy source on the growth and glutamic acid uptake by different species of streptococci (35 strains from 14 species) and their adaptation to glucose and lactose. Our experiments show the special behaviour of S. thermophilus: lactose is seen to be better substrate than glucose for both bacterial growth and active transport. AlI the other streptococci tested, except perhaps S. salivarius, prefer glucose or use both sugars in the same way. A previous adaptation of group N streptococci enhances the use of this disaccharide. According to these data, we have classified the streptococci studied in three groups.
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