The inhibition of Listeria monocytogenes in milk by dehydrated pediococcal spent medium

Abstract

A dehydrated pediococcal spent medium (DPSM) from each Pediococcus pentosaceus L7230 and P. acidilactici Pa was used against four strains of Listeria monocytogenes (CA, OH, SA and V7). L7230 DPSM (2 mg per drop) formed zones of inhibition against all the tested Listeria strains. The growth preventing concentration (GPC) of L7230 DPSM against the strains of Listeria (10 5 cfu ml −1 ) in milk (4·5°C) was 15 mg ml −1 . The antilisterial activity of sub GPC levels of L7230 DPSM in milk (32 and 4·5°C) increased with the increase in concentration. Levels of 5 and 7·5 mg L7230 DPSM ml −1 were bacteriostatic during the first 3 days against both Listeria CA and SA. Concentrations of 5, 7·5 and 10 mg ml −1 were bactericidal against the Listeria strains after day 3, and from the start of the testing period, respectively. In many instances, the concentrations of 7·5 and 10 mg ml −1 reduced the listerial populations below 1·0 log 10 The concentration of 10 mg ml −1 had the same bactericidal effect at both 32 and 4·5°C against Listeria OH. In general, the D 7·5 values, of those Listeria strains showing biphasic curves of their populations in milk, were higher during phase I (1·9–5·4 days) than those in phase II (0·8–1·2 days). Listeria SA had the highest D 7·5 value of 5·8 days which was consistent throughout the 6 day exposure period. The control preparations (all at 10 mg ml −1 milk) used in this study such as P. acidilactici Pa DPSM, and each uninoculated lyophilized broth (ULB, pH 6·6 and 5·1) allowed L. monocytogenes strain OH to grow (32°C) to the same extent as it would in milk with no added preparation.