Superoxide dismutase protects Escherichia coli against killing by human serum.

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.