Survival assay for E. coli under abiotic stresses

E. coli cells were cultivated to the stage (OD₆₀₀ = 1.0) at 37 °C in LB medium and then transferred into fresh LB medium with 100 mg/mL ampicillin and 1 mM IPTG. After expression induced for 2 h, the cultures were shifted to 50 °C for thermotolerance assays. One ml culture samples were respectively taken at 0, 1, 2 and 3 h and then their serial dilutions were plated in triplicate. Cell viability was evaluated by counting average colony-forming units from triplicate plates. For salt stress treatment, aliquots from IPTG-induced cultures were treated with 800 mM NaCl for 1, 2 or 3 h, and then were plated on LB medium. For both heat and salt treatments, E. coli cells transformed with empty pET32a(+) vector (Novagen, Merck, Darmstadt, Germany) were used as the control. The photographs of colony formation were taken with after culture on plates for 12 h at 37 °C.