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Background  Lipolysis is an important process of cheese ripening that contributes to the formation of flavour.  Propionibacterium freudenreichii  is the main agent of lipolysis in Emmental cheese; however, the enzymes involved produced by this species have not yet been identified. Lipolysis is performed by esterases (carboxylic ester hydrolases, EC 3.1.1.-) which are able to hydrolyse acylglycerols bearing short, medium and long chain fatty acids. The genome sequence of  P. freudenreichii  type strain CIP103027 T  was recently obtained in our laboratory.  The aim of this study was to identify as exhaustively as possible the potential esterases in  P. freudenreichii  that could be involved in the hydrolysis of acylglycerols in Emmental cheese. The proteins identified were produced in a soluble and active form by heterologous expression in  Escherichia coli  for further study of their activity and specificity of hydrolysed substrates.    Results  The approach chosen was a genomic search approach that combined and compared four methods based on automatic and manual searches of homology and motifs among  P. freudenreichii  CIP103027 T  predicted proteins. Twenty-three putative esterases were identified in this step. Then a selection step permitted to focus the study on the 12 most probable esterases, according to the presence of the GXSXG motif of the α/β hydrolase fold family. The 12 corresponding coding sequences were cloned in expression vectors, containing soluble N-terminal fusion proteins. The best conditions to express each protein in a soluble form were found thanks to an expression screening, using an incomplete factorial experimental design. Eleven out of the 12 proteins were expressed in a soluble form in  E. coli  and six showed esterase activity on 1-naphthyl acetate and/or propionate, as demonstrated by a zymographic method.    Conclusion  We were able to demonstrate that our genomic search approach was efficient to identify esterases from the genome of a  P. freudenreichii  strain, more exhaustively than classical approaches. This study highlights the interest in using the automatic search of motifs, with the manual search of homology to previously characterised enzymes as a complementary method. Only further characterisations would permit the identification of the esterases of  P. freudenreichii  involved in the lipolysis in Emmental cheese.

A genomic search approach to identify esterases in Propionibacterium freudenreichii involved in the formation of flavour in Emmental cheese