Brasca Milena, Hogenboom Johannes A, Morandi Stefano, Rosi Veronica, D'Incecco Paolo, Silvetti Tiziana, Pellegrino Luisa
Institute of Sciences of Food Production, National Research Council of Italy , Milan, Italy.
Department of Food, Environmental and Nutritional Sciences, University of Milan , Milan, Italy.
J Agric Food Chem. 2016 Nov 16;64(45):8604-8614. doi: 10.1021/acs.jafc.6b03403. Epub 2016 Nov 8.
A set of 191 strains of Streptococcus thermophilus were preliminarily screened for the presence of the genes codifying for cell envelope-associated proteinase (prtS) and for glutamate decarboxylase (gadB) responsible for γ-aminobutyric acid (GABA) production. The growth and proteolytic activity of the gadB-positive strains (9 presenting the prtS gene and 11 lacking it) were studied in microfiltered pasteurized milk. Degradation of both caseins (capillary electrophoresis) and soluble nitrogen fractions (HPLC) and changes in the profile of free amino acids (FAAs; ion-exchange chromatography) were evaluated at inoculation and after 6 and 24 h of incubation at 41 °C. None of the strains was capable of hydrolyzing caseins and β-lactoglobulin, and only two hydrolyzed part of α-lactalbumin, these proteins being present in their native states in pasteurized milk. Contrarily, most strains were able to hydrolyze peptones and peptides. For initial growth, most strains relied on the FAAs present in milk, whereas, after 6 h, prtS strains released variable amounts of FAA. One prtS strain expressed a PrtS phenotype, and two prtS strains showed a rather intense proteolytic activity. Only five strains (all prtS) produced GABA, in variable quantities (up to 100 mg/L) and at different rates, depending on the acidification strength. Addition of glutamate did not induce production of GABA in nonproducing strains that, however, unexpectedly were shown to adopt the degradation of arginine into citrulline and ornithine as an alternative acid resistance system and likely as a source of ATP.
对一组191株嗜热链球菌进行了初步筛选，以检测编码与细胞包膜相关蛋白酶（prtS）以及负责γ-氨基丁酸（GABA）产生的谷氨酸脱羧酶（gadB）的基因。研究了gadB阳性菌株（9株含有prtS基因，11株不含该基因）在微滤巴氏杀菌乳中的生长和蛋白水解活性。在接种时以及在41℃培养6小时和24小时后，评估了酪蛋白（毛细管电泳）和可溶性氮组分（高效液相色谱）的降解以及游离氨基酸（FAA；离子交换色谱）谱的变化。没有一株菌株能够水解酪蛋白和β-乳球蛋白，只有两株水解了部分α-乳白蛋白，这些蛋白质在巴氏杀菌乳中以天然状态存在。相反，大多数菌株能够水解蛋白胨和肽。对于初始生长，大多数菌株依赖于乳中存在的FAA，而在6小时后，prtS菌株释放出不同量的FAA。一株prtS菌株表现出PrtS表型，两株prtS菌株表现出相当强的蛋白水解活性。只有五株菌株（均为prtS）产生GABA，产量各不相同（高达100 mg/L），且速率不同，这取决于酸化强度。添加谷氨酸不会在不产生GABA的菌株中诱导GABA的产生，然而，出乎意料的是，这些菌株被证明采用将精氨酸降解为瓜氨酸和鸟氨酸作为替代的酸抗性系统，并且可能作为ATP的来源。
