Isolation of Lactococcus lactis strains producing inhibitory activity against Listeria

Fifty-one bacteriocin-producing bacteria (Bac(+)) were isolated from 35 raw milk cheeses by the deferred antagonism method using Listeria strains as indicator organisms. Of the 51 Bac(+) isolates, 10 that were identified as Lactococcus lactis subsp. lactis by biochemical and physiological tests, and were selected on the basis of the inhibition zone diameter (>10 mm). Five lactococcal strains containing the nisA or nisZ genes, the structural genes for nisin biosynthesis, were selected with specific PCR amplifications, and the other five isolates were shown to produce a bacteriocin different from nisin. Seven of the ten isolates including two presumptive nisogenic strains, showed good capacity for milk acidification decreasing the pH of milk from 6.5 to 4.7-4.3.