Salmonella Typhimurium: Heat-shocked cells vs. control cells and acid-shocked cells vs. control cells

We investigated if the transcriptional response of S. Typhimurium to temperature and acid variations was hysteretic and showed memory, i.e., if the transcriptional regulation caused by environmental stimuli remained after the stimuli ceased. The transcriptional activity of non-replicating stationary-phase cells of Salmonella Typhimurium caused by the exposure to 45°C and to pH 5 for 30 min was monitored by microarray hybridizations before the application of the stress and at the end of the treatment period, as well as immediately and 30 minutes after conditions were set back to their initial values, 25°C and pH 7, respectively. We also investigated if hysteresis was accompanied with higher resistance to inactivation conditions, 50°C and/or pH 3, as well as more efficient growth at extreme conditions, 43°C and/or pH 4.5. RNA samples for gene expression analysis and culture samples for growth and inactivation experiments were obtained after 30 min with stress (45°C or pH 5), immediately after the removal of stress and 30 min after the removal of stress. A control culture at 25°C and pH 7 was sampled at the same time intervals. Three independent replicates were carried out.