Nisin-resistant (Nis r ) strains of Clostridium botulinum and Listeria monocytogenes may arise as nisin becomes more widely used as an additional safety barrier in minimally-processed foods. The sensitivity of Nis r L. monocytogenes ATCC 700301 and ATCC 700302 and toxigenic Nis r C. botulinum 169B to low pH, salt, sodium nitrite, and potassium sorbate was assayed using discontinuous gradients in broth and compared to the parental wild-type strains. The nisin-resistant strains did not have intrinsic resistance to low pH, sodium chloride, potassium sorbate, or sodium nitrite. In no case were the Nis r L. monocytogenes and C. botulinum strains examined more resistant to inhibitors than the parental strains.
K . D . M O D I , M . L . C H I K I N D A S A N D T . J . M O N T V I L L E . 2000.Nisin, a bacteriocin produced by some strains of Lactococcus lactis, acts against foodborne pathogen Listeria monocytogenes. A single exposure of cells to nisin can generate nisin-resistant (Nis r ) mutants, which may compromise the use of nisin in the food industry.The objective of this research was to compare the heat resistance of Nis r and wild type (WT) Listeria monocytogenes.The synergistic e¡ect of heat-treatment (55 C) and nisin (500 IU ml À1 ) on the Nis r cells and the WT L. monocytogenes Scott Awas also studied. When the cells were grown in the absence of nisin, there was no signi¢cant (a 0Í05) di¡erence in heat resistance betweenWTand Nis r cells of L. monocytogenes at 55, 60 and 65 C. However, when the Nis r cells were grown in the presence of nisin, they were more sensitive to heat at 55 C than the WT cells.The D-values at 55 C were 2Í88 and 2Í77 min for Nis r ATCC 700301and ATCC 700302, respectively, which was signi¢cantly (a 0Í05) lower than the D-value for WT, 3Í72 min.When Nis r cells were subjected to a combined treatment of heat and nisin, there was approximately a four log reduction during the ¢rst 7 min of treatment.
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