Torrie J P, Cholette H, Froehlich D A, McKellar R C
J Dairy Res. 1983 Aug;50(3):365-74. doi: 10.1017/s0022029900023207.
An extracellular proteinase-and lipase-deficient mutant of a psychrotroph, Pseudomonas fluorescens strain 32A, has been isolated and the absence of the proteinase enzyme confirmed by growth on differential media, enzyme assay and polyacrylamide gel electrophoresis. Competition between the parent and the mutant was observed when equal numbers of the 2 strains were inoculated together into raw skim-milk at 6 degrees C. Bitterness was detected at 6 degrees C in pasteurized skim-milk inoculated with the parent cells concurrent with the detection of proteolytic activity. In the case of the mutant, slight bitterness which did not increase with increasing cell numbers was detected in the absence of proteolysis. Mutant cells failed to grow on Na caseinate as the sole source of carbon. It was concluded that the extracellular proteinase, while not essential for growth in milk, does provide a selective advantage to the producer organism. This enzyme is, however, essential for growth on milk proteins and contributes to the development of bitterness in pasteurized milk.
已分离出一种嗜冷菌荧光假单胞菌32A株的胞外蛋白酶和脂肪酶缺陷型突变体，并通过在鉴别培养基上生长、酶活性测定和聚丙烯酰胺凝胶电泳证实了该蛋白酶的缺失。当将等量的这两种菌株一起接种到6℃的生脱脂乳中时，观察到亲本菌株和突变体之间的竞争。在用亲本细胞接种的巴氏杀菌脱脂乳中，在6℃时检测到苦味，同时检测到蛋白水解活性。就突变体而言，在没有蛋白水解的情况下检测到轻微苦味，且苦味不会随着细胞数量的增加而增加。突变体细胞不能以酪蛋白酸钠作为唯一碳源生长。得出的结论是，胞外蛋白酶虽然不是在牛奶中生长所必需的，但确实为产生该酶的生物体提供了选择性优势。然而，这种酶对于在牛奶蛋白上生长是必需的，并且有助于巴氏杀菌牛奶中苦味的产生。
