title: The transcriptional heat shock response of Salmonella typhimurium shows hysteresis and heated cells show increased resistance to heat and acid stress.
abstract: We investigated if the transcriptional response of Salmonella Typhimurium to temperature and acid variations was hysteretic, i.e. whether the transcriptional regulation caused by environmental stimuli showed memory and remained after the stimuli ceased. The transcriptional activity of non-replicating stationary phase cells of S. Typhimurium caused by the exposure to 45 °C and to pH 5 for 30 min was monitored by microarray hybridizations 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. One hundred and two out of 120 up-regulated genes during the heat shock remained up-regulated 30 minutes after the temperature was set back to 25 °C, while only 86 out of 293 down regulated genes remained down regulated 30 minutes after the heat shock ceased. Thus, the majority of the induced genes exhibited hysteresis, i.e., they remained up-regulated after the environmental stress ceased. At 25 °C the transcriptional regulation of genes encoding for heat shock proteins was determined by the previous environment. Gene networks constructed with up-regulated genes were significantly more modular than those of down-regulated genes, implying that down-regulation was significantly less synchronized than up-regulation. The hysteretic transcriptional response to heat shock was accompanied by higher resistance to inactivation at 50 °C as well as cross-resistance to inactivation at pH 3; however, growth rates and lag times at 43 °C and at pH 4.5 were not affected. The exposure to pH 5 only caused up-regulation of 12 genes and this response was neither hysteretic nor accompanied of increased resistance to inactivation conditions. Cellular memory at the transcriptional level may represent a mechanism of adaptation to the environment and a deterministic source of variability in gene regulation.
authors.affiliation: Institute of Food Research, Norwich, United Kingdom. carmen.pin@bbsrc.ac.uk
authors.forename: Carmen
authors.initials: C
authors.lastname: Pin
authors.forename: Trine
authors.initials: T
authors.lastname: Hansen
authors.forename: Marina
authors.initials: M
authors.lastname: Muñoz-Cuevas
authors.forename: Rob
authors.initials: R
authors.lastname: de Jonge
authors.forename: Jesper T
authors.initials: JT
authors.lastname: Rosenkrantz
authors.forename: Charlotta
authors.initials: C
authors.lastname: Löfström
authors.forename: Henk
authors.initials: H
authors.lastname: Aarts
authors.forename: John E
authors.initials: JE
authors.lastname: Olsen
mesh_terms: D000704:Analysis of Variance; D021301:Antifreeze Proteins / Q000378:metabolism*; D005786:Gene Expression Regulation / Q000502:physiology*; D018869:Heat-Shock Response / Q000502:physiology*; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D046228:Microarray Analysis; D017510:Protein Folding; D012486:Salmonella typhimurium / Q000254:growth & development*; D014158:Transcription, Genetic / Q000502:physiology*
chemical_list: D021301:Antifreeze Proteins