La biosynthèse des acides aminés à chaîne branchée et des purines : deux voies essentielles pour une croissance optimale de Streptococcus thermophilus dans le lait

Abstract

Branched-chain amino acids and purine biosynthesis: two pathways essential for optimal growth of Streptococcus thermophilus in milk. Slowing down and delays in Streptococcus thermophilusgrowth in milk have negative effects for dairy technology. Our objectives are to under- stand the reasons for those delays, and to identify some of the functions essential for optimal growth of S. thermophilusin milk. To reach this goal, a random insertional mutagenesis was performed on a S. thermophilus strain chosen for its ability to grow rapidly in milk. We obtained 14 different mutants, affected in their capacity to grow quickly in milk. Four of them were further character- ized: two were mutated in the branched-chain amino acids biosynthesis operon and two others were mutated in the purine nucleotides biosynthesis operon. Consequently, branched-chain amino acids and purine biosynthesis pathways are functional in S. thermophilus. Our results suggested that the amount of both branched-chain amino acids and purine nucleotides, present in milk, and directly consumable is not sufficient for an optimal growth. Moreover, we identified the branched-chain amino acids and purine biosynthesis pathways as essential functions for optimal growth of S. thermophilusin milk. milk / Streptococcus thermophilus / growth / branched chain amino acid / purine