Fig. 1

Activation of chaperon system and their regulating sigma factors in Escherichia coli cells due to heat shock. rpoS encoded alternative sigma factor σ^S, which acts as the master regulator of the general stress response is usually activated at the early stationary phase at any temperature (even below 37 °C). Up to the early stationary phase, usually cells are observed as viable and culturable (as shown in green box); however, during the entry into the stationary phase, a significant fraction of the total cell population become viable but nonculturable (VBNC) due to the accumulation of the reactive oxygen species (ROS). The figure shows the possible recruitment of the heat shock proteins (HSPs) in association of different sigma factors under heat stress. The rpoD encoded housekeeping sigma factor or the primary sigma factor σ^D together with σ^S transcribing most of the genes in growing cells are activated at 37–41 °C, a temperature range where cells are presumably found at their maximal growing state. Transcription of the rpoH gene for σ^H is induced at elevated temperature (generally at 42 °C or more) via the action of σ^E. Both σ^E and σ^H act as the regulator of the extra-cytoplasmic/extreme HSPs (DnaKJ, OMP, PPiD, GroEL, ClpX, GrpE). The HSPs are expected to be activated at 42–48 °C, where cells are usually completely non-culturable. Correlated reference of the narrated temperatures can be found in the text