Stress response variability can cause up to 3-fold increase in the thermal resistance of Salmonella strains

The variability in the bacterial stress response has received plenty of attention during the last years, partly due to its relevance to microbial risk assessment. Although the microbial response is affected by numerous variability sources, previous studies focused mostly on strain variability (inherent differences between strains of the same bacterial species) under optimal growth conditions. Here, we analyze a variability source relatively unexplored within microbial risk assessment: stress response variability. This refers to physiological differences due to prior exposure to stressful environments. Namely, we studied the impact of sub-optimal pre-culture conditions or the application of an acid shock on the thermal resistance of two strains of Salmonella (a reference strain and a highly heat resistant one). We observed that stress response variability is strain dependent. The heat resistance of the reference strain had a significant increase in heat resistance (up to 3-fold increase), whereas the conditions tested resulted in a reduction of thermal resistance with respect to control conditions (up to 2-fold reduction). Considering that magnitude of these changes are comparable to strain variability, and that stress response variability might be common throughout the food supply chain, this study evidences the need to study this phenomenon further in order to incorporate it into quantitative microbial risk assessments.