Changes in cellular structure of heat-treated Salmonella in low-moisture environments

Salmonella cells desiccated in an environment with low-water activity (a_w ) show longer survival times and enhanced resistance to heat. However, little is known about the cellular ultrastructure of Salmonella in low-a_w environment in relation to the survival and persistence during desiccation.

In this study, Salmonella Enteritidis strain PT30 was dehydrated by exposure to air or by mixing with wheat flour (a_w 0·30 at room temperature) for 7 days followed by heat treatment at 80°C for 10, 20, 60 min respectively. Transmission electron microscopy (TEM) was employed to examine and compare the ultrastructure of heat-treated S. Enteritidis cells after desiccation with the cells suspended in trypticase soy broth (TSB). Cells suspended in TSB broth showed disrupted ribosomes, congregated proteins and denatured DNA. However, no significant alterations were observed in the ultrastructure of the desiccated cells after heat treatment. The number of desiccated S. Enteritidis cells decreased by <1·5 log CFU per gram after 80°C treatment for 60 min, however, cells suspended in TSB declined more than 5 log₁₀ CFU per mL at 80°C within 5 min.

A drastic difference in the number of survivors and cellular ultrastructure was observed between vegetative and air or food-dried S. Enteritidis cells after subjecting to heat treatment at 80°C. No significant ultrastructure changes were observed in desiccated cells after heat treatment except for roughening and corrugating surfaces.

This study provides a direct comparison to illustrate how desiccation influences the cell ultrastructure before/after heat treatment, which will aid in better understanding of the fundamental mechanism underlying the increased thermal resistance of Salmonella cells in low-a_w environment.