Strains of Escherichia coli with sufficient K antigen to resist killing by complement were poorly phagocytosed when injected intravenously into mice. Phagocytosis was markedly increased by anti-OK but not by anti-O sera. In contrast anti-K sera had little or no effect on the bactericidal reaction. This was not because K antigenic sites were scarce but may have been because their position was such that complement was activated at a distance from its substrate. Red cells coated with K antigen were poorly lysed by complement and anti-K serum, suggesting that the K antibody did not activate complement very effectively although again the sites may have been too superficial. The effect of K antigens on phagocytosis and complement killing or lysis could all be explained by their ability to impair protein binding.
Strains of E. coli rich in K antigen were resistant to phagocytosis and complement killing and were virulent for mice on intracerebral injection. The significance of K antigens in animal and human infections is discussed.