Development of a Probability Model to Describe the Uncertainty of the Time to Inactivation of Salmonella enterica under a Desiccated Environment

Introduction: During the process of bacterial cell inactivation, frequency distribution of the time to inactivation occurs due to the probabilistic nature of the event. The variability of bacterial survival is apparently observed in a small population. Less than 100 viable cells of Salmonella enterica are sufficient for the infection of humans. Estimating the time to inactivation probabilistically is useful to determine a realistic estimation of the risk of the pathogenic bacteria. We focused on low water activity foods as critical pathogen carriers.

Purpose: We aimed to develop a predictive model to describe the variability of the time to inactivation of Salmonella enterica Typhimurium under a desiccated environment through a bacterial experiment and a computer simulation.

Methods: In the bacterial experiment, aliquots of 2 μl of a Salmonella Typhimurium suspension were placed into a 96-well microplate, and the microplate was then placed in a drying chamber (10-20% RH) at different temperatures ranging from 5°C to 25°C. The survival of the cells in each well was confirmed by the turbidity after adding nutrient broth (100 μl). In computer simulation, the time of inactivation of the bacterial populations was estimated by random sampling from individual cells in the kinetics model. 

Results: The survival probability was estimated by the number of the surviving wells in the 96-well microplate and was described as a cumulative gamma distribution. In addition, the variability of the time to inactivation of bacteria was described as a gamma distribution. The larger the initial bacterial numbers are, the wider the range of time to inactivation observed in both the experiment and the computer simulation.

Significance: The probability model could be used for describing the variability of the time to inactivation of bacterial population, which would play an important role in assessing the risk of low water activity foods.