Changes of cell permeability during Listeria monocytogenes persistence under nisin treatment

The study examined free radical production of Listeria monocytogenes under nisin stress by colorimetric cell proliferation assay, and used flow cytometry (FC) to investigate the persister population of L. monocytogenes since agar plating is limited in providing information on persister cells exposed to nisin. The stationary phase populations in spent medium or resuspended in fresh TSB medium were individually treated with 75 μg/ml nisin for 24 hr at 30°C. Intracellular and extracellular free radical production was detected in the resuspended group simultaneously, while free radicals were only detected within cells in the spent medium group during the nisin treatment, and FC indicated possible population differentiation from quadrants H1-1 (“dead region,”) H1-2 (“damaged region) and H1-3(“live region”). The change in cell permeability was different under the two conditions (spent medium vs. fresh medium) when facing exposure to nisin, indicating the effect of the environment on cell persistence under nisin stress. Practical applications The practical application of this work is an awareness of how the sensitivity of a food isolate of L. monocytogenes exposed to nisin as a food preservative, and survival of tested strains can be affected by the environment in generating a population of persistent cells. A nutrient rich environment appears to enhance the population of persister cells. This is an important consideration for the food industry trying to control L. monocytogenes in a nutrient rich environment such as food. In addition, FC can be used to rapidly assess the population of persister cells compared with dead cells and provide a useful tool in risk assessment of L. monocytogenes in a food where nisin is used as a preservative.