A Pathogen Survival Kinetics Model for a Low-water Activity Environment and in Low-water Activity Foods

Introduction:  Food poisoning induced by pathogenic bacteria occasionally occurs through the ingestion of low water activity (a_w) foods. Salmonella enterica and pathogenic Escherichia coli are highly tolerant to desiccation compared with other bacterial species. Modeling the survival kinetics of both bacterial species as a function of a_w and temperature could be useful for microbial risk assessments for these bacteria.

Purpose:  This study aimed to develop predictive models that can be used to estimate the number of bacteria in low a_w environmental conditions and low a_w foods.

Methods:  We investigated the survival kinetics of 4 serotypes of Salmonella enterica (Stanley, Typhimurium, Chester, and Oranienburg) and 3 serotypes of Escherichia coli (O26, O111, and O157) when exposed to a combination of three temperatures (5°C, 15°C, and 25°C) and 5 a_w conditions (0.22, 0.43, and 0.93). The bacterial cultures (20 µl) were placed onto sterile plastic plates and dried in a safety cabinet for 4 hours. After drying, the plates were placed in plastic containers that were adjusted to each a_w condition using saturated salt solutions. In addition to these in vitro experiments, we examined the survival kinetics of these bacterial species on the surfaces of chocolate, cheddar cheese, almonds, and radish sprout seeds under various temperatures.

Results:  Regardless of the a_w and the serotype, a rapid decrease in the number of viable bacterial cells was observed at 25°C compared with 5°C. The survival kinetics followed a Weibull distribution. The estimated Weibullian model parameters were described as a function of temperature but not of a_w.

Significance:  The results of this study enable the prediction of the number of pathogenic bacteria in low a_w foods under various storage temperatures. This information could contribute to assessing the risk of these bacteria in low a_w foods.