Persister cell formation of Listeria monocytogenes in response to natural antimicrobial agent nisin

The persistence of Listeria monocytogenes (L. monocytogenes) can be defined by its prolonged existence in food environments that poses a risk to food safety. The persistence of cells exposed to antibiotics and chemical sanitisers is well recorded however the persistence following treatment with natural antimicrobials like bacteriocins has not been determined. This study used two L. monocytogenes isolates, one from food and one from an animal origin, to obtain a persistent subpopulation following exposure to the natural antimicrobial nisin. Overnight cultures of L. monocytogenes were treated with 75 μg/ml of nisin over 24 h, and a biphasic killing pattern was observed for both strains. Persister cells became a population of cells showing tolerance to the high concentrations of nisin in our tests and proved not to be resistant with nisin re-exposure tests. XTT assays showed that the reduced level of metabolism maybe an important requirement for the L. monocytogenes persistence and the persister population which could survive at high concentration of nisin was probably related to less negative cell surface charges. The current data will be initial evidence to understand the mechanism of persistence through gene expression and in the future to give more reliable clues in developing strategies to control this food safety risk.