Control of Listeria monocytogenes on ready-to-eat ham and fresh cut iceberg lettuce using a nisin containing Lactococcus lactis fermentate

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Listeria monocytogenes is a potentially deadly psychrotrophic pathogen that can cause illness through contamination of ready-to-eat (RTE) foods. In this work we evaluated the use of a fermentate consisting of cells and supernatant of a nisin producing Lactococcus lactis strain to control L. monocytogenes in foods. In addition, to determine the role of nisin production in this activity, a transposon screen was carried out to isolate a nisin-negative mutant. Several independent mutants containing transposon insertions in the nisin gene cluster were obtained which exhibited a complete loss of anti-L. monocytogenes activity in an agar-based assay. A simple media using food grade components was found to support strong growth and nisin production of Lc. lactis 537 following incubation at between 20 and 35 °C. The ability of the fermentate to control L. monocytogenes in foods was evaluated in cut iceberg lettuce and RTE sliced ham stored at 4 °C for 10 days. Initial levels of 103–104 CFU/g L. monocytogenes were reduced to below detection limit (<20 CFU/g) immediately by the fermentate and remained so for the duration of the trial. In comparison, fermentate from the nisin-negative nisB mutant was unable to reduce L. monocytogenes levels initially. It did however provide some L. monocytogenes growth inhibition in ham and faster killing in lettuce at later time points, but not to undetectable levels. These results demonstrate that nisin containing fermentate provides a simple and effective means to control L. monocytogenes on RTE foods.