Development of a Probability Model to Describe the Variability in the Time to Inactivation of Salmonella enterica

Introduction: Despite the development of many predictive models, variability in time to inactivation of bacterial populations still cannot be precisely estimated. It is difficult to evaluate the risk of surviving bacteria in foods after inactivation treatment, although a few remaining bacterial cells, such as Salmonella enterica or enterohaemorrhagic Escherichia coli, can cause foodborne infection. Accurate estimation of death probability for a bacterial population would be useful for determining the risk of the pathogenic bacteria. This is particularly important in low water activity foods.

Purpose: This study was undertaken to develop a probability model to estimate the death probability of a S. enterica Typhimurium population under conditions of desiccation over time.

Methods: Bacterial cells were prepared with an initial concentration adjusted to 1×10ⁿ CFU/μl (where n=1, 2, 3, 4, 5) by 10-fold dilution. The inocula was dispensed into 96-well microplates (2 µl per well). The microplates were stored under desiccating conditions (10 to 20% relative humidity) at 5, 15, or 25°C. The survival of bacterial cells in each well was assessed by adding 100 μl of tryptic soy broth as an enrichment culture at arbitrary time intervals.

Results: The changes in the death probability of 96 replicate bacterial populations were described as cumulative gamma distribution. Variability in time to inactivation was described to transform a cumulative gamma distribution into a gamma distribution. In addition, the certainty levels were described for bacterial inactivation that ensure death probability of a bacterial population at six different levels, from 90 to 99.9999%.

Significance: The study results supported the use of the model for describing death probability of a bacterial population. Death probability is useful in risk assessments that estimate bacteria remaining after inactivation processes.