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To assess the role of superoxide dismutase in protecting Escherichia coli from killing by human serum and neutrophils, we constructed isogenic, smooth-lipopolysaccharide K-12 strains, either sod wild-type, delta sodA, or delta sodA delta sodB. The delta sodA delta sodB strain was killed by serum much more readily than either the wild-type or delta sodA strain. After allowing for this serum sensitivity difference, the delta sodA delta sodB strain also showed increased susceptibility to phagocytic killing by human neutrophils. These results indicate that superoxide dismutase protects E. coli from killing by serum (complement system) and by human neutrophils, possibly by a role in maintaining bacterial membrane structure.

Redox-cycling agents catalyze the flow of reducing equivalents to oxygen; this process generates superoxide ion and other reduced oxygen species. Measurements of redox-cycling activity have been performed previously by studying cyanide-resistant oxygen consumption (respiration) of Escherichia coli cells. E. coli strain GK100, lacking both terminal oxidases, has almost no measurable respiration. We show that the use of this strain eliminates the requirement for cyanide in measurements of redox-cycling activity. The addition of either menadione sodium bisulfite or plumbagin, well-known redox-cycling agents, to GK100 cells resulted in high levels of oxygen consumption. The rate of menadione bisulfite-induced oxygen consumption in this respiration-deficient strain, in the absence of cyanide, was comparable to the cyanide-resistant respiration of isogenic respiration-proficient E. coli strains. In GK100 cells, cyanide increased menadione bisulfite-induced oxygen consumption but had no effect on plumbagin-induced oxygen consumption.