Relationship between glass transition temperature, and desiccation and heat tolerance in Salmonella enterica

Pathogenic bacteria such as Salmonella enterica exhibit high desiccation tolerance, enabling long-term survival in low water activity (a(w)) environments. Although there are many reports on the effects of low a(w) on bacterial survival, the mechanism by which bacteria acquire desiccation tolerance and resistance to heat inactivation in low-a(w) foods remains unclear. We focused on the glass transition phenomenon, as bacteria may acquire environmental tolerance by state change due to glass transition. In this study, we determined the glass transition temperature (T(g)) in S. enterica serovars under different a(w) conditions using thermal rheological analysis (TRA). The softening behaviour associated with the state change of bacterial cells was confirmed by TRA, and T(g) was determined from the softening behaviour. T(g) increased as the a(w) decreased in all S. enterica serovars. For example, while the T(g) of five S. enterica serovars was determined as 35.16°C to 57.46°C at 0.87 a(w), the T(g) of all the five serovars increased by 77.10°C to 83.30°C at 0.43 a(w). Furthermore, to verify the thermal tolerance of bacterial cells, a thermal inactivation assay was conducted at 60°C for 10 min under each a(w) condition. A higher survival ratio was observed as a(w) decreased; this represented an increase in T(g) for Salmonella strains. These results suggest that the glass transition phenomenon of bacterial cells would associate with environmental tolerance.