Stress responses of thermophilic Geobacillus sp. NTU 03 caused by heat and heat-induced stress.

A batch culture of Geobacillus sp. NTU 03 was subjected to a rapid temperature shift for investigating the stress response. Several known heat-shock responses for protein, DNA, and cell membrane recurring were observed on two-dimensional (2D) gels. Heat caused protein and cell membrane disruption greatly affected the electron transport chain. Further, heat caused lower dissolved oxygen (DO) solubility resulting in insufficient oxygen to be electron acceptor, and the NADH could not be reoxidized. Hence, we observed seven dehydrogenase that used NADH as electron donor were downregulated on the 2D gels. In contrast, succinate dehydrogenase that used FADH(2) as electron donor was upregulated. However, this induction may simultaneously increase generation of superoxide; therefore the cellular redox state was imbalanced. We observed that superoxide dismutase (2D gel) and zinc ion ABC transporter (mRNA quantification) were upregulated, whereas ferric ion ABC transporter (2D gel and mRNA quantification) was downregulated. Increase in the reactive oxygen or nitrogen species scavenging activities were also observed. For responding the lower DO solubility, a transient activation of nitrate respiration was observed at transcriptional level. Our results support the view that both heat stress and heat-induced stress should be considered together when investigating the stress responses of thermophiles.