Little is understood about the impact of environmental conditions on the virulence plasticity of Listeria monocytogenes strains grown in food. In this report, we monitored changes in the virulence properties of one high virulent (CCUG 3998) and one low virulent (442) L. monocytogenes strains grown on raw salmon (Salmo salar L.). The effect of temperature exposures (0 degrees C, 4 degrees C and 20 degrees C) on the expression levels of virulence genes (hlyA, actA, inlA and prfA), invasion into Caco-2 cells and in vivo mouse infection was analysed. Our results showed that L. monocytogenes virulence genes are differentially expressed when salmon is stored at different temperatures. Of the four virulence genes, the transcript levels for inlA were strongly affected, which correlated with the strain's virulence capacity as assessed by Caco-2 cells. In contrast to CCUG 3998, the virulence of strain 442 was altered with tested conditions. This strain maintains its low virulence status as far as salmon is stored at lower temperatures, but increases its virulence at higher temperatures. These results lead to the indication that exposure to abuse temperature conditions might influence the virulence potential of low pathogenic L. monocytogenes strains in salmon.
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