M . M AN S OU R, D . A MR I , A . B O UT TE F RO Y, M . L IN D ER AN D J. B. M IL LI E RE . 1999. The effects of nisin and monolaurin, alone and in combination, were investigated on Bacillus licheniformis spores in milk at 37°C. In the absence of inhibitors, germinated spores developed into growing vegetative cells and started sporulation at the end of the exponential phase. In the presence of nisin (25 IU ml −1 ), spore outgrowth was inhibited (4 log 10 reduction at 10 h). Regrowth appeared between 10 and 24 h and reached a high population level (1·25 × 10 8 cfu ml −1 ) after 7 d. Monolaurin (250 mg ml) had a bacteriostatic effect during the first 10 h but thereafter, regrowth occurred slowly with a population level after 7 d (4 × 10 5 cfu ml) lower than that of nisin. Different combined effects of nisin (between 0 and 42 IU ml ) were investigated using a Doehlert matrix in order to study the main effects of these factors and the different interactions. Results were analysed using the Response Surface Methodology (RSM) and indicated that nisin and monolaurin had no action on spores before germination; only pH values had a significant effect (P ¾ 0·001), i.e. spore count decreased as the pH value increased in relation to germination. Sublethal concentrations of nisin (30 IU ml ) and monolaurin (100 mg ml −1 ) in combination acted synergistically on outgrown spores and vegetative cells, showing total inhibition at pH 6·0, without regrowth, within 7 d at 37°C.
Strain C19, isolated from fermented cucumbers and identified as Lactobacillus plantarum, produced a bacteriocin. This bacteriocin, named plantaricin C19, was stable at acidic pH, was relatively thermostable, and had a molecular mass of about 3.5 kDa. It inhibited some pathogenic (i.e., Listeria spp.) and spoilage Gram-positive bacteria but had weak or no action against lactic acid bacteria. Its adsorption on sensitive and resistant bacteria was pH dependent and was reduced by pretreatment of cells with lipase or lysozyme. Curing treatments with acriflavine or novobiocin yielded nonproducing mutants sensitive to plantaricin C19.Key words: lactic acid bacteria, Lactobacillus plantarum, bacteriocin, protein.
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