OLIGOPEPTIDES ARE THE MAIN SOURCE OF NITROGEN FOR LACTOCOCCUS-LACTIS DURING GROWTH IN MILK

The consumption of amino acids and peptides was monitored during growt h in milk of proteinase-positive (Prt(+)) and -negative (Prt(-)) strai ns of Lactococcus lactis, The Prt(-) strains showed monophasic exponen tial growth, while the Prt(+) strains grew in two phases. The first gr owth phases of the Prt(+) and Prt- strains were the same, and no hydro lysis of casein was observed, Also, the levels of consumption of amino acids and peptides in the Prt(+) and Prt(-) strains were similar. At the end of this growth phase, not all free amino acids and peptides we re used, indicating that the remaining free amino acids and peptides w ere unable to sustain growth. The consumption of free amino acids was very low (about 5 mg/liter), suggesting that these nitrogen sources pl ay only a minor role in growth. Oligopeptide transport-deficient strai ns (Opp(-)) of L. lactis were unable to utilize oligopeptides and grew poorly in milk, However, a di- and tripeptide transport-deficient str ain (DtpT(-)) grew exactly like the wild type (Opp(+) DtpT(+)) did, Th ese observations indicate that oligopeptides represent the main nitrog en source for growth in milk during the first growth phase. In the sec ond phase of growth of Prt(+) strains, milk proteins are hydrolyzed to peptides by the proteinase, Several of the oligopeptides formed are t aken up and hydrolyzed internally by peptidases to amino acids, severa l of which are subsequently released into the medium (see also E. R. S . Kunji, A. Hagting, C. J. De Vries, V.Juillard, A. J. Haandrikman, B. Poolman, and W. N. Konings, J. Biol. Chem. 270:1569-1574, 1995). It i s concluded that growth of L. Iactis in milk depends on oligopeptides as nitrogen source for 98% of growth and that the oligopeptide transpo rt system plays a crucial role in the utilization of these peptides.