UTILIZATION OF NITROGEN-SOURCES DURING GR OWTH OF LACTOCOCCUS-LACTIS IN MILK

Lactococci have very limited capacities of synthesizing amino acids an d therefore must utilize exogenous nitrogen sources for optimal growth . Although lactococci possess several distinct amino acid transport sy stems, this nitrogen source plays a minor role during the growth proce ss in milk. The use of the free amino acids of the milk accounts for o nly 2% of the total growth. Peptide utilization by lactococci mainly d epends on their translocation abilities. Three different peptide trans port systems have been identified in lactococci. However, only the oli gopeptide transport system (Opp) is absolutely necessary for the use o f the milk peptides as nitrogen source. Endogenous milk oligopeptides account for 8% of the total growth. Therefore, caseins represent the m ain source of amino acids. Casein utilization first requires a cell en velope-located proteinase, which releases only oligopeptides from case ins. Several of these oligopeptides are smalt enough to be translocate d inside the cell via the Opp system. All the essential amino acids ar e present in these small oligopeptides, which therefore can supply the amino acid requirements of the cell. it is concluded that the Opp sys tem plays a crucial role during the growth process in milk. It represe nts the main pathway for amino acid supply during growth in milk, sinc e oligopeptides, whatever their origin (ie, initially present in milk or derived from caseins), account for 98% of the growth.