Disclosed is a pO157 plasmid-specified polypeptide found in E. coli EDL933 and other E. coli that binds to and cleaves C1-esterase inhibitor, and antibodies specific for the polypeptide. Also disclosed are methods employing the polypeptide for diagnosing enterohemorrhagic E. coli infection, identifying potential inhibitors of its activity, and reducing viscosity of material containing glycosylated polypeptides.