FIG. 6.

Role for K. pneumoniae YmdF in resistance to oxidative stress. (A) Tolerance responses of class I mutants to various stresses. Bacterial cultures of the wild type and each of the class I mutants, which were grown in LB medium at 37°C, were adjusted to a turbidity value of 1 and were then subjected to treatment with 20 mM of H₂O₂ for 2 h at 37°C. The bacterial concentration upon oxidative stress was determined using a MicroScan turbidity meter (Dade Behring, CA). Results shown are means ± standard deviations for three independent experiments. * indicates statistical significance as determined by Student's t test (P < 0.05). (B) Northern blot analysis of ymdF transcripts. Total RNAs were isolated from wild-type K. pneumoniae grown in LB, in M9-glucose, and under various stress conditions. Twenty micrograms of total RNA from each sample was subjected to Northern blotting, and a biotin-labeled ymdF RNA probe was used for the hybridization. The expression levels of ymdF upon exposure to H₂O₂ for 1, 3, 6, 9 min; treatment with 15 mM paraquat (PQ); or treatment with different concentrations of sucrose for 15 min are shown in lanes 1 to 8, and the abundances of ymdF transcripts in K. pneumoniae grown in LB or M9-glucose medium is shown in lanes 9 to 16.