| Summary: | Human listeriosis, caused by the consumption of food contaminated with Listeria monocytogenes, is on the top five most commonly reported zoonosis under the surveillance of the European Union (EU), and presents the highest fatality rate. This foodborne pathogen is particularly problematic for the food industry because it is widespread in the environment, and because of its ability to survive under several stress conditions such as refrigeration temperatures, high salt concentrations, low water activity (aw), or a wide pH range. Exposure to a single or multiple sublethal stresses, as those impaired by food processing and food matrices, can enhance tolerance of Listeria monocytogenes to stresses and increase its survival and pathogenesis. This knowledge is needed to develop efficient control strategies to improve food safety. The main objective of this study was to investigate and compare the impact of exposure to stress conditions, frequently encountered in food-associated environments, on the virulence potential of eight L. monocytogenes strains, including strains EDG-e, Scott A and Lm 2542, associated with a listeriosis outbreak occurred in Portugal. Strains were grown in Brain Heart Infusion (BHI) medium (i) at low temperature (11 ºC, cold stress), (ii) supplemented with 6% NaCl (w/v) at low temperature (11 ºC; cold-osmotic stress), and (iii) at standard conditions (37 ºC), and tested for their ability to invade the human intestinal epithelial Caco-2 cells. Our results demonstrated that long-term exposure to cold stress conditions contributed for enhanced invasion efficiency of the four tested strains when compared to cells grown at 37 ºC, while no differences were observed when cells were grown at high salt concentrations. Further experiments evaluated the effect of exposure to these stress conditions on the survival of three selected L. monocytogenes strains through an in vitro gastrointestinal (GI) tract digestion model, using Ultra High Temperature (UHT) milk as food matrix, and subsequent invasiveness potential. The exposure to cold-osmotic stress increased the survival of one L. monocytogenes strain (Lm 2542) through the GI tract, that subsequently presented a significantly higher invasion efficiency. More studies are necessary for a better understanding of the mechanisms that overlap between adaptation to stress improving and an increase in virulence-related characteristics in these specific strains of L. monocytogenes.
|
|---|