Fig. 4.

Comparison of the oxidative stress tolerances of napA mutants. RDIT9.32 (wild-type), RWY10.3 (ΔnapA) and RWY17.3 (OE : : napA) strains were incubated at 37 °C for 5 days, and selected culture media were supplemented with 200 µg pyridoxine l⁻¹, as required. Stress-initiated percentage changes (ΔA) in growth (colony diameters) were determined for each strain and stress-generating agent, and differences between the percentage changes recorded for the mutants and the RDIT9.32 control strain (ΔΔA = ΔA_mutant−ΔA_RDIT9.32) were calculated. Significantly decreased stress tolerances were found in oxidative stress (MSB, tBOOH and H₂O₂)-exposed RWY10.3 (ΔnapA) cultures (−80 % <ΔΔA <−20 %); meanwhile, growth stimulation caused by the overexpression of napA (RWY17.3) was ΔΔA ≤15 % under different oxidative stress treatments. Pyridoxine supplementation increased slightly the oxidative stress resistance of RDIT9.32 (ΔΔA ≤5 %), which was similar to previous findings (Balázs et al., 2010).