Cell structure degradation in Escherichia coli and Thermococcus sp. strain Tc-1-95 associated with thermal death resulting from brief heat treatment

The thermal death mechanism of microorganisms when heated at lethally high temperatures is still not fully understood. In this study, we examined the relationship between thermal death and degradation of the cell structure in the mesophilic bacterium Escherichia coli strain W3110 and the hyperthermophilic archaeon Thermococcus sp. strain Tc-1-95. By heating the microorganisms at lethally high temperatures only briefly (1.5 s duration) in a flow-type apparatus, we studied the microbial cells at very early and critical stages of the thermal death process. For E. coli, it was found that the loss of viability was not associated with thermal damage to the cell envelope. Deformation of the nucleoid was observed. These results suggest that the thermal death of E. coli is attributed to thermal denaturation or degradation of cytoplasmic molecules. On the other hand, the thermal death of Thermococcus sp. strain Tc-1-95 was strongly associated with rupture of the cell envelope. Furthermore, massive deformation of the S-layer with lethal thermal stress was observed. These results demonstrate that the thermal deaths of the two microorganisms investigated proceed via very different mechanisms. The contrast can be attributed to the difference in their cell envelope structures.