Sensitivity of nisin-resistant Listeria monocytogenes to heat and the synergistic action of heat and nisin

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 effect of heat-treatment (55°C) and nisin (500 IU ml^-1) on the Nis(r) cells and the WT L. monocytogenes Scott A was also studied. When the cells were grown in the absence of nisin, there was no significant (α = 0.05) difference in heat resistance between WT and 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 2.88 and 2.77 min for Nis(r) ATCC 700301 and ATCC 700302, respectively, which was significantly (α = 0.05) lower than the D-value for WT, 3.72 min. When Nis(r) cells were subjected to a combined treatment of heat and nisin, there was approximately a four log reduction during the first 7 min of treatment.