Involvement of a prophage in the lysis of Lactococcus lactis subsp. cremoris AM2 during cheese ripening

Lactococcus lactis subsp. cremoris AM2 strain was previously shown to lyse early and extensively during cheese ripening. This strain is lysogenic and contains a prophage named Phi AM2. Lysis of strain AM2 and its prophage-cur ed derivative AM2-C in Saint-Paulin pressed-type cheese was monitored by th e following parameters: cell viability, morphological changes of bacteria o bserved by electron microscopy and release of intracytoplasmic peptidases. Proteolysis was quantified by measuring soluble nitrogen (SN), phosphotungs tic acid soluble nitrogen (PTA-N) and free amino acids. By contrast to the wild type strain AM2 which lyses early and extensively, its prophage-cured derivative AM2-C lyses only slowly and to a limited extent in cheese. These results indicate that the prophage Phi AM2 is involved in the lytic behavi our of L. lactis AM2 during cheese ripening. In addition, the comparison of two isogenic strains with similar enzymatic potential but different abilit y to lyse demonstrates that starter strain lysis results in a higher free a mino acids production rate and a decrease of bitter taste. (C) 1998 Elsevie r Science Ltd. All rights reserved.