Fig. 4.

Adaptation of wild-type YPH250 cells to H₂O₂ challenge requires nutrient-rich medium to stimulate catalase activity. Catalase activity (A) and cell viability (B) without (control) and with pre-adaptation of cells to H₂O₂ followed by challenge with a lethal H₂O₂ dose. Exponentially growing cells in YPD medium for 12 h (OD₆₀₀=0.5) were pre-challenged with 0.2 mM H₂O₂ for 1 h in YPD or in KPi followed by challenge with 2 mM H₂O₂ in KPi for 1.5 h (see Scheme 1). Note that control cells were incubated for 1.5 h (YPD/KPi) and 2.5 h (KPi) in the absence of glucose and had likely started to respire, which may explain their higher catalase activity relative to control cells in Fig. 2A. Additionally, the switch of cells from YPD to KPi increases catalase activity of control cells as shown in Fig. 3A. Cell viability and catalase activity were measured as described under Materials and methods and results are the average ±SD for three independent cultures (n=3).