Complex formation between Escherichia coli lipopolysaccharide O antigen and capsular K antigen as detected by immunoelectrophoresis.

Many Escherichia coli of serotypes commonly found in the normal intestine and in extraintestinal diseases, and having capsular antigens of the low molecular group called group II, will, in simple saline extracts, produce complexes between some or all of the lipopolysaccharide molecules and some of the polysaccharide K molecules. Non-complex-forming and complex-forming strains with the same O and K can be found. The complexes are thermostable but are disrupted by some detergents. O-K complex formation may lead to misinterpretation of immunoprecipitation results; one example is the counter current technique used for K determination of E. coli. In this technique O antigen lipopolysaccharide may, when complexed to K polysaccharide, mimic a K antigen. The possible implications of O-K complex formation during the infection process, especially for antibody formation need to be examined.