Superoxide level in E. coli. E. coli (lpp-deleted) was transformed with pQE-sodA and pQE-ygfZ to express recombinant SodA and YgfZ, respectively, and the superoxide levels in bacteria were determined by monitoring the fluorescence changes after loading with dihydroethidium . Data were taken after 120-min treatments with chemicals. (A) Both paraquat (50 μM) and plumbagin (50 μM) stimulated the levels of superoxide detected. (B) The superoxide stimulation seen in (A) was suppressed by SodA expression. (C) The same experiments in (B) were repeated with bacteria expressing YgfZ. Note: pQE60 was the vector control.