Utilisation des sources d'azote du lait par Lactococcus lactis

Abstract

Lactococci have very limited capacities of synthesizing amino acids and therefore must utilize exogenous nitrogen sources for optimal growth. Although lactococci possess several distinct amino acid transport systems, this nitrogen source plays a minor role during the growth process in milk. The use of the free amino acids of the milk accounts for only 2% of the total growth. Peptide utilization by lactococci mainly depends on their translocation abilities. Three different peptide transport systems have been identified in lactococci. However, only the oligopeptide transport system (Opp) is absolutely necessary for the use of the milk peptides as nitrogen source. Endogenous milk oligopeptides account for 8% of the total growth. Therefore, caseins represent the main source of amino acids. Casein utilization first requires a cell envelope-located proteinase, which releases only oligopeptides from caseins. Several of these oligopeptides are small enough to be translocated inside the cell via the Opp system. All the essential amino acids are present in these small oligopeptides, which therefore can supply the amino acid requirements of the cell. It is concluded that the Opp system plays a crucial role during the growth process in milk. It represents the main pathway for amino acid supply during growth in milk, since oligopeptides, whatever their origin (ie, initially present in milk or derived from caseins), account for 98% of the growth.