Listeriolysin S may inhibit the anti-listerial properties of Lactobacillus plantarum.

Listeriosis is a serious infection linked to the consumption of food contaminated with Listeria monocytogenes. Outbreaks and mortality rates associated with this infection make it a significant public health concern. As biocontrol agents, probiotics such as Lactobacillus plantarum had been of interest for the promotion of antilisterial activities. However, a recent bacteriocin from epidemic L. monocytogenes strains called listeriolysin S (LLS) has been identified with the ability to target the prokaryotic cells that may hinder the anti-listerial properties of L. plantarum. The present study was designed to investigate the interplay between serotypes 4b (lineage I, LLS-producing strain) and 1/2a (NCTC7973, lineage II, non LLS-producing strain) L. monocytogenes and L. plantarum ATCC13643. According to the results of the co-culture assay, L. plantarum significantly reduced the growth of LLS- L. monocytogenes. However, there was a significant reduction in the growth of L. plantarum when co-cultured with LLS + L. monocytogenes. Moreover, according to the results of the culture assay using Caco-2 cell line, there was a significant reduced intracellular count of LLS- L. monocytogenes after L. plantarum exposure, whereas, no major differences were observed in the intracellular count of LLS + L. monocytogenes. These results suggest that L. plantarum may be unable to inhibit infections caused by LLS-producing L. monocytogenes. Also, phylogenetic studies showed the presence of LLS-like proteins in several environmental isolates including L. innocua which suggests a role for LLS in survival and bacterial colonization in harsh conditions. In overall, the ability of LLS to target certain bacterial cells should be taken into consideration during the development of anti-listerial probiotics. Future experiments are required to elucidate the exact mechanisms by which LLS achieves bacterial killing.