Some physical properties of K antigens of Escherichia coli related to their biological activity.

The ability of K antigens from Escherichia coli to inhibit the agglutination of sheep red cells by rabbit antibody has been taken as a measure of the ability of such antigens to render strains of E. coli O-inagglutinable, resistant to complement and to phagocytosis, and to increase their virulence for mice. Analysis of K antigens by chromatography on Sepharose and diethylaminoethyl cellulose columns and by immunoelectrophoresis suggested that agglutination-inhibiting activity was directly related to molecular size rather than charge. The effect of alkali treatment showed inhibitory activity to be independent of K-specific antigenic determinants and confirmed its relation to molecular weight.