Can a sugar-binding protein in human blood help kill E. coli in laboratory tests?

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Laboratory experiments tested the complement-dependent bacteria-killing activity of serum mannan-binding protein (MBP) against rough strains of Escherichia coli K-12 and B. MBP is a lectin—a sugar-binding protein—specific for mannose and N-acetylglucosamine and is known to activate complement through the classical pathway.

The bacteria were first sensitized with purified human serum MBP in the presence of Ca2+ and then incubated with guinea pig complement. They showed a marked decrease in colony-forming ability compared with bacteria not sensitized with the lectin. The bactericidal effect depended on the concentrations of both the lectin and complement. The reaction's dependence on C4 indicated that MBP's complement-dependent bactericidal action operated through the classical pathway. The lectin also caused the bacteria to aggregate.

Scatchard plot analysis of 125I-labeled MBP binding to the bacteria gave a dissociation constant (Kd) of 6 × 10⁻⁹ M and a maximum binding capacity of 30,000 MBP molecules per cell. Binding was inhibited by mannose, N-acetylglucosamine, N-acetylmannosamine, L-fucose, manno-heptulose, and sedoheptulose. This suggested that MBP recognized L-glycero-D-manno-heptose and N-acetylglucosamine in the core oligosaccharide of the E. coli K-12 cell wall, and L-glycero-D-manno-heptose in E. coli B.

The findings suggest a physiological role for this serum lectin in host defense and are consistent with the lack of virulence of rough E. coli strains in mammals.