Factors involved in bactericidal activity of sheep serum

The bactericidal activity of blood, plasma, and serum from sheep was measured with 3 strains of Escherichia coli. Plasma showed greater bactericidal activity than blood or serum for strains O78:K80(B) and Lilly. The antibody-complement system of normal serum killed the rough strain, Lilly, but neither antibody nor complement alone were bactericidal. Antibody, complement, and an additional heat-stable cofactor were necessary for killing strain O78:K80(B). Strain 3662 was not susceptible to the lethal action of the antibody-complement system. Capsular and flagellar antibodies did not appear to play any part in the bactericidal activity of serum. There was no relationship between the titer of antibodies to the somatic antigen and the bactericidal titer in the immune serums. The antibodies necessary for killing the smooth strain of E coli, O78:K80(B), were absorbed with the rough strain, Lilly, but not by the other smooth strain, 3662. It is suggested that the target antigen of the bactericidal antibody is not the somatic antigen, but a rough antigen internal to the O polysaccharide.