title: The fast milk acidifying phenotype of Streptococcus thermophilus can be acquired by natural transformation of the genomic island encoding the cell-envelope proteinase PrtS.
abstract: BACKGROUND
  In industrial fermentation processes, the rate of milk acidification by Streptococcus thermophilus is of major technological importance. The cell-envelope proteinase PrtS was previously shown to be a key determinant of the milk acidification activity in this species. The PrtS enzyme is tightly anchored to the cell wall via a mechanism involving the typical sortase A (SrtA) and initiates the breakdown of milk casein into small oligopeptides. The presence or absence of PrtS divides the S. thermophilus strains into two phenotypic groups i.e. the slow and the fast acidifying strains. The aim of this study was to improve the milk acidification rate of slow S. thermophilus strains, and hence optimise the fermentation process of dairy products.
  
  
  RESULTS
  In the present work, we developed for the first time a strategy based on natural transformation to confer the rapid acidification phenotype to slow acidifying starter strains of S. thermophilus. First, we established by gene disruption that (i) prtS, encoding the cell-envelope proteinase, is a key factor responsible for rapid milk acidification in fast acidifying strains, and that (ii) srtA, encoding sortase A, is not absolutely required to express the PrtS activity. Second, a 15-kb PCR product encompassing the prtS genomic island was transferred by natural transformation using the competence-inducing peptide in three distinct prtS-defective genetic backgrounds having or not a truncated sortase A gene. We showed that in all cases the milk acidification rate of transformants was significantly increased, reaching a level similar to that of wild-type fast acidifying strains. Furthermore, it appeared that the prtS-encoded activity does not depend on the prtS copy number or on its chromosomal integration locus.
  
  
  CONCLUSIONS
  We have successfully used natural competence to transfer the prtS locus encoding the cell-envelope proteinase in three slow acidifying strains of S. thermophilus, allowing their conversion into fast acidifying derivatives. The efficient protocol developed in this article will provide the dairy industry with novel and optimised S. thermophilus starter strains.
authors: Biochimie et Génétique Moléculaire Bactérienne, Institut des Sciences de la Vie, Université catholique de Louvain, Place Croix du Sud 5/L7.07.06, B-1348 Louvain-la-Neuve, Belgium.
authors: Damien
authors: D
authors: Dandoy
authors: Christophe
authors: C
authors: Fremaux
authors: Marie Henry
authors: MH
authors: de Frahan
authors: Philippe
authors: P
authors: Horvath
authors: Patrick
authors: P
authors: Boyaval
authors: Pascal
authors: P
authors: Hols
authors: Laetitia
authors: L
authors: Fontaine
mesh_terms: D000818:Animals; D001426:Bacterial Proteins / Q000235:genetics* / Q000378:metabolism*; D002417:Cattle; D003611:Dairy Products; D005285:Fermentation; D044404:Genomic Islands; D006801:Humans; D019344:Lactic Acid / Q000378:metabolism*; D008892:Milk / Q000378:metabolism* / Q000382:microbiology*; D010641:Phenotype; D012697:Serine Endopeptidases / Q000235:genetics* / Q000378:metabolism*; D014170:Transformation, Genetic
chemical_list: D001426:Bacterial Proteins; D019344:Lactic Acid; C417204:PrtS protein, Streptococcus thermophilus; D012697:Serine Endopeptidases