Growth of Listeria monocytogenes in Milk and its Control by Pediocin 5 Produced by Pediococcus acidilactici UL5

Pediocin 5, a bacteriocin produced by Pediococcus acidilactici UL5 in MRS medium, displayed a bactericidal effect on three Listeria monocytogenes strains grown in partly skimmed milk containing 1% milk-fat (MF) and 3.25% MF homogenized milk at 4 degrees C. Viable counts of all Listeria inoculated into 1% MF milk without pediocin 5 gradually increased from 10(7) to 10(8)-10(9) cfu/ml after 13 days storage. Addition of pediocin 5 to the milk led to a sharp reduction in viable Listeria by approximately 3.0 log cycles during the first day, followed by a slow growth of the survivors to reach 5 x 10(5)-10(6) cfu/ml after 13 days. However, at an inoculum level of about 10(3) cfu/ml, survival of the three Listeria was below the detection limit of the method (2-3 cfu/ml) in 1% MF milk containing pediocin 5 during 8 days storage; populations reached approximately 10 cfu/ml after 13 days, whereas they reached 10(4)-10(5) cfu/ml in the absence of pediocin 5. Growth behavior in 3.25% MF milk was similar to that in 1% MF milk at the two inoculum levels with or without pediocin 5. Occasional repeated contamination by Listeria was simulated by five successive inoculations of 1% MF milk every 48 h after the first inoculation (each corresponding to approximately 2 x 10(2) cfu/ml of milk). After 11 days storage, only about 50 viable Listeria per ml were found in milk containing pediocin 5, although control without pediocin 5 reached 10(3)-10(4) cfu/ml. Increased pediocin 5 concentration in milk led to decreased survival of L. monocytogenes Lm21.