Invasiveness of Listeria monocytogenes strains of Caco-2 cells in response to a period of extreme salt stress reflecting salt-curing and rehydration of cod (Gadus morhua L.)

Products like salt-cured fish contain approximately 15-21% NaCl and are rehydrated to 2-3% NaCl before preparation and consumption. These products are regarded as safe, but it has been shown that Listeria spp. is able to survive at extreme levels of salt and start to grow after rehydration. Thus, the ability of salt stressed Listeria monocytogenes to cause listeriosis, measured as its ability to invade Caco-2 cells was studied in this paper. Seven strains of L. monocytogenes and one Listeria innocua were cultivated in BHI to early and late stationary phase at 4 °C. At both phases, the strains were exposed to either no salt or to salt stress comparable to that applied in the production of rehydrated salt-cured cod, i.e. 21% NaCl followed by dilution to 2% NaCl. In addition, the eight strains were cultivated in BHI with 2% NaCl, which is similar to the salt content as in rehydrated salt-cured cod and other ready-to-eat (RTE) products as well. The ability of non salt stressed L. monocytogenes strains to enter Caco-2 cells was significant higher (p > 0.05) compared to the corresponding strains exposed to 21% NaCl for 96 h, followed by 2% NaCl for 48 h. On the other hand, L. monocytogenes cultivated in BHI with 2% NaCl showed a higher invasiveness of Caco-2 cells than both the other sample categories. As the ability to invade Caco-2 cells correlates with bacterial virulence, the results suggests that L. monocytogenes represent a lower food safety risk when exposed to salt-curing with extreme NaCl concentrations than exposure of a constant and moderate level of salt commonly used in RTE products.