Different epithelial cell response to membrane vesicles produced by Listeria monocytogenes cultured with or without salt stress

We have previously shown that Listeria monocytogenes, a causative agent of listeriosis, can produce membrane vesicles (MVs)during in vitro culture. The aim of this study was to investigate the ability of MVs from L. monocytogenes cultured with or without salt stress to induce cytotoxicity and pro-inflammatory responses in colon epithelial Caco-2 cells. MVs were purified from wild-type L. monocytogenes 10403S strain and an isogenic ΔsigB mutant strain. MVs from both wild-type and ΔsigB mutant strains increased viability of Caco-2 cells regardless of salt stress. Both MVs from wild-type and ΔsigB mutant strains stimulated expression of pro-inflammatory cytokine and chemokine genes in Caco-2 cells. Expression levels of pro-inflammatory cytokine genes in cells treated with MVs from bacteria cultured without salt stress were significantly higher than those in cells treated with MVs from bacteria cultured with salt stress. However, expression levels of chemokine genes in cells treated with MVs from bacteria cultured with salt stress were significantly higher than those in cells treated with MVs from bacteria cultured without salt stress. In addition, expression levels of interleukin (IL)-1β and IL-8 genes were partially inhibited by either lysozyme-treated MVs or ethylenediaminetetraacetic acid-treated MVs compared to those after treatment with intact MVs. Our results suggest that salt stress can affect the production of L. monocytogenes MVs, thus causing different pro-inflammatory responses in host cells.