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The bactericidal and opsonic activities of normal sheep serum against 9 smooth and 4 rough strains of gram-negative bacteria were measured. Three smooth strains-Escherichia coli 3662, Salmonella typhimurium, and Salmonella gallinarum-were resistant to the bactericidal action of normal sheep serum with or without complement. Six smooth strains-E coli O78:K80(B), Salmonella arizonae, Proteus inconstans, Klebsiella pneumoniae, Salmonella stanley, and Salmonella abortus-equi-were moderately sensitive to bactericidal activity of fresh sheep serum. After complement was added, serum heated at 56 C for 30 minutes did not regain its bactericidal activity against E coli O78:K80(B), P inconstans, nor S abortus-equi. Four rough strains-E coli Lilly, Salmonella cholerae-suis, Shigella sonnei, and Hafnia alvei-were very sensitive to the bactericidal activity of normal sheep serum. Somatic agglutinins were demonstrated in all of the sheep serums against all of the bacteria, but this antibody activity was partially destroyed by heating at 56 C for 30 minutes. The opsonic activity of normal sheep serum was demonstrated in normal mice. In the mouse peritoneal cavity, normal sheep serum enhanced opsonic activity against all smooth strains and 1 rough strain of the bacteria. The other 3 rough strains were extremely sensitive to the opsonic system of normal mice.

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.

A survey was conducted on the level of conglutinin (K) and immuno-conglutinin (IK) in the sera of apparently healthy man and animals. The conglutinating activity of sheep, goats, pigs, fowls and humans was entirely due to IK. Cattle and buffaloes had both K and IK. Conglutinin was present only in Zebu cattle and buffaloes. There was neither K nor IK in the horses. Human sera had a very low level of IK.