title: Proteolytic Activity and Production of γ-Aminobutyric Acid by Streptococcus thermophilus Cultivated in Microfiltered Pasteurized Milk.
abstract: 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.
authors: Institute of Sciences of Food Production, National Research Council of Italy , Milan, Italy.
authors: Milena
authors: M
authors: Brasca
authors: Department of Food, Environmental and Nutritional Sciences, University of Milan , Milan, Italy.
authors: Johannes A
authors: JA
authors: Hogenboom
authors: Institute of Sciences of Food Production, National Research Council of Italy , Milan, Italy.
authors: Stefano
authors: S
authors: Morandi
authors: Department of Food, Environmental and Nutritional Sciences, University of Milan , Milan, Italy.
authors: Veronica
authors: V
authors: Rosi
authors: Department of Food, Environmental and Nutritional Sciences, University of Milan , Milan, Italy.
authors: Paolo
authors: P
authors: D'Incecco
authors: Institute of Sciences of Food Production, National Research Council of Italy , Milan, Italy.
authors: Tiziana
authors: T
authors: Silvetti
authors: Department of Food, Environmental and Nutritional Sciences, University of Milan , Milan, Italy.
authors: Luisa
authors: L
authors: Pellegrino
mesh_terms: D000596:Amino Acids / Q000378:metabolism; D000818:Animals; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D002364:Caseins / Q000378:metabolism; D002417:Cattle; D006868:Hydrolysis; D007768:Lactalbumin / Q000378:metabolism; D007782:Lactoglobulins / Q000378:metabolism; D008892:Milk / Q000737:chemistry / Q000382:microbiology*; D061352:Pasteurization; D048249:Streptococcus thermophilus / Q000201:enzymology / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D005680:gamma-Aminobutyric Acid / Q000096:biosynthesis*
chemical_list: D000596:Amino Acids; D001426:Bacterial Proteins; D002364:Caseins; D007782:Lactoglobulins; D005680:gamma-Aminobutyric Acid; D007768:Lactalbumin
keywords: GABA; Streptococcus thermophilus; arginine; citrulline; free amino acids; microfiltered milk; ornithine; proteolysis