Cell-wall proteinases PrtS and PrtB have a different role in Streptococcus thermophilus/Lactobacillus bulgaricus mixed cultures in milk

• Published in: Microbiology (Society for General Microbiology) Vol. 148; no. 11; pp. 3413 - 3421
• Main Authors: Courtin, P, Monnet, V, Rul, F
• Format: Journal Article
• Language: English
• Published: Reading Soc General Microbiol 01.11.2002; Society for General Microbiology; Microbiology Society
• Subjects: Animals; Bacterial Proteins; Biological and medical sciences; Cell Wall - enzymology; Culture Media; Endopeptidases - genetics; Endopeptidases - metabolism; Endopeptidases - physiology; Food industries; Fundamental and applied biological sciences. Psychology; Hydrogen-Ion Concentration; Lactobacillus - growth & development; Lactobacillus - physiology; Lactobacillus bulgaricus; Life Sciences; Milk - metabolism; Milk and cheese industries. Ice creams; PrtB protein; prtS gene; PrtS protein; Serine Endopeptidases - genetics; Serine Endopeptidases - physiology; Streptococcus - growth & development; Streptococcus - physiology; Streptococcus thermophilus; Lactic acid bacteria; Microorganism growth; Enzyme; Mixed microorganism culture; Fermentation starter; Cell wall; Streptococcaceae; Lactobacillus delbrueckii subsp. bulgaricus; Peptidases; Streptococcus salivarius subsp. thermophilus; Bacteria; Hydrolases; Lactobacillaceae; Micrococcales; Milk; bacterial growth; thermophilic bacteria; milk fermentation
• ISSN: 1350-0872; 1465-2080
• DOI: 10.1099/00221287-148-11-3413

Unité de Biochimie et Structure des Protéines, INRA, 78352 Jouy-en-Josas Cedex, France 1 Author for correspondence: F. Rul. Tel: +33 1 34 65 21 48. Fax: +33 1 34 65 21 63. e-mail: rul{at}jouy.inra.fr The manufacture of yoghurt relies on the simultaneous utilization of two starters: Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus ( Lb. bulgaricus ). A protocooperation usually takes place between the two species, which often results in enhanced milk acidification and aroma formation compared to pure cultures. Cell-wall proteinases of Lactococcus lactis and lactobacilli have been shown to be essential to growth in milk in pure cultures. In this study, the role of proteinases PrtS from S. thermophilus and PrtB from Lb. bulgaricus in bacterial growth in milk was evaluated; a negative mutant for the prtS gene of S. thermophilus CNRZ 385 was constructed for this purpose. Pure cultures of S. thermophilus CNRZ 385 and its PrtS-negative mutant were made in milk as well as mixed cultures of S. thermophilus and Lb. bulgaricus : S. thermophilus CNRZ 385 or its PrtS-negative mutant was associated with several strains of Lb. bulgaricus , including a PrtB-negative strain. The pH and growth of bacterial populations of the resulting mixed cultures were followed, and the Lactobacillus strain was found to influence both the extent of the benefit of Lb. bulgaricus / S. thermophilus association on milk acidification and the magnitude of S. thermophilus population dominance at the end of fermentation. In all mixed cultures, the sequential growth of S. thermophilus then of Lb. bulgarius and finally of both bacteria was observed. Although proteinase PrtS was essential to S. thermophilus growth in milk in pure culture, it had no effect on bacterial growth and thus on the final pH of mixed cultures in the presence of PrtB. In contrast, proteinase PrtB was necessary for the growth of S. thermophilus , and its absence resulted in a higher final pH. From these results, a model of growth of both bacteria in mixed cultures in milk is proposed. Keywords: bacterial growth, milk fermentation, thermophilic bacteria Abbreviations: FSDA, Fast Slow Difference Agar; LAB, lactic acid bacterium/bacteria