Lactococcin MMT24, a novel two-peptide bacteriocin produced by Lactococcus lactis isolated from rigouta cheese

Lactococcin MMT24 is a novel bacteriocin produced by Lactococcus lactis MMT24, a strain isolated from a Tunisian traditional cheese. The bacteriocin shows a narrow antimicrobial activity against closely related lactic acid bacteria. Lactococcin MMT24 is heat resistant, remains active after incubation at pH 3 to 10, lyophilization, long-term storage at -20 degrees C and is sensitive to treatment with proteolytic enzymes. The mode of action of lactococcin MMT24 was identified as bactericidal. Purification of the active compound showed that lactococcin MMT24 consists of two distinct peptides, named pepalpha and pepbeta, whose complementary action is necessary for full antibacterial activity. Optimal antibacterial activity was obtained when the complementary peptides pepalpha and pepbetawere present in equal amounts. Mass spectrometry analysis showed masses of 3765.33 Da and 3255.26 Da for pepalpha and pepbeta, respectively. These molecular masses do not correspond to those of so far described bacteriocins. Addition of 50 nmol l(-1) of lactococcin MMT24 to cells of L. lactis ssp. cremoris ATCC11603 induced increase in the concentration of K+ in supernatant indicating a massive leakage of this ion from the cells. This release was most likely caused by pores formation by the pepalphaand pepbeta peptides in the target bacterial membrane.