The sensitivity to complement of strains of Escherichia coli related to their K antigens.

We have confirmed that K antigens influence the sensitivity to complement of strains of . Resistant strains bound more polycation and by inference therefore had a higher surface negative charge than sensitive strains. Extracts containing K antigen non-specifically inhibited red cell agglutination and this inhibitory activity was roughly proportional to complement resistance. All of five resistant strains became more sensitive to complement when grown at unusual temperatures and extracts from them then had less inhibitory activity. In four strains of serotype O6 K13 complement resistance was proportional to K antigen content measured by immunodiffusion. However, purified K antigen from a resistant strain (WF82) had much greater agglutination inhibiting activity weight for weight than purified K antigen from a sensitive strain (WF96). In experiments with I-labelled haemolysin K antigens decreased the binding of both IgG and IgM antibodies and also directly reduced complement activity. The mechanisms of action of K antigens and their relation to virulence are discussed.