LAUSR

Purpose. This study aimed to evaluate the effect of two types of stress, cold and nutritional, on the viability and the in vitro virulence of the foodborne pathogenic bacteria Listeria monocytogenes. Methodology. Ten diverse isolates were kept in phosphate‐buffered saline (PBS) at optimal (37 °C) or at refrigeration temperature (7 °C), for 1 and 7 days. The viability of the cells [log colony‐forming units (c.f.u.)/ml] and their in vitro virulence, before and after storage in these conditions, were investigated. In vitro virulence (log PFA) was evaluated using the human intestinal epithelial cell line HT‐29 in plaque‐forming assays (PFAs). Results/Key findings. In general, when compared with the conditions at 37 °C, the exposure at 7 °C for 7 days seemed to increase the resistance of the isolates to nutritional stress. Nutritional stress per se acted significantly to decrease the in vitro virulence of the isolates. After 7 days of nutrient deprivation, whether at optimal or at refrigeration temperature, the majority of the isolates assumed a low‐virulence phenotype. Conclusion. Our results suggest that when L. monocytogenes are in refrigerated post‐processing environments that are unable to support their growth they may increase their resistance to nutritional stress and may decrease their virulence. This should be considered when performing risk assessments for refrigerated ready‐to‐eat (RTE) foods.