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, ΔsodA, or ΔsodAΔsodB. The ΔsodAΔsodB strain was killed by serum much more readily than either the wild-type or ΔsodA strain. After allowing for this serum sensitivity difference, the ΔsodAΔsodB 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.