Listeria monocytogenes and Salmonella enterica affect the expression of nisin gene and its production by Lactococcus lactis.

The Lactococcus lactis is known as a probiotic bacterium and also as a producer of nisin. Nisin has been approved by related legal agencies to be used as an antimicrobial peptide in food preservation. In fact, the L. lactis is present in different food products along with other micro-organisms especially pathogenic bacteria. So, it is important to predict the behavior of nisin-producer strain in contact with other pathogens. In this regard, nisin gene expression and the level of secreted biologically active form of nisin by L. lactis subsp. lactis in modified MRS broth and whey solution in co-culture with Listeria monocytogenes or Salmonella enterica were studied. The nisin concentration was determined by microbiological assay method and the transcription level of nisin gene was assayed through quantitative reverse transcription PCR (RT-qPCR). According to our results, the highest concentration of nisin and its gene transcription level were detected in mono- and co-cultures after 16 h of incubation, concurrent with the end of L. lactis exponential phase of growth. The nisin mRNA copies in co-cultures were higher than mono-cultures only at 16 h of incubation. But, differences between nisin concentrations in mono- and co-cultures were significant at 16, 24 h and at 12, 16, 24 h of incubation in the modified MRS medium and whey solution, respectively. This incompatibility could be related to the low availability of components required for nisin precursor modification, transportation and processing in mono-cultures. Overall, the L. lactis produced more mature and active nisin when it was in contact with pathogenic bacteria.