Abstract
Proteolytic and acidifying properties of Streptococcus thermophilus strains isolated from yoghurt or cheeses were evaluated. Among 30 strains tested, 12 exhibited cell envelope-associated proteinase activity (PrtS +), three displayed a slight PrtS activity (PrtS +/−) and 15 were PrtS −, despite the presence of the corresponding gene ( prtS) in eight of them. Sequencing of the prtS gene in four PrtS − and one PrtS + strains revealed that the absence of PrtS activity in the PrtS − strain probably results from an alteration of the prtS regulation. The strains displaying the highest acidifying capacities were all PrtS +. All but one PrtS + strains were phylogenetically close, as shown by the sequencing of their rDNA internal transcribed spacer (ITS) 16S-23S. More specifically, the high proteolytic and acidifying capacities are associated with the presence of a type II-ITS.
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