Serum factors responsible for killing of Shigella

Abstract

Eight strains of Shigella were tested for susceptibility to killing by seven normal human sera. Although there was a wide range of susceptibility between strains of bacteria, there was surprisingly little difference in the killing activity of individual sera and no relationship between antibody titres and killing capacity.

Bacteriolysis required small amounts of antibody, but as little as 0.02 mg of a 19S fraction from normal serum restored full killing capacity to 1 ml of antibody depleted serum. Neither 11S IgA nor Cohn fraction II restored the bacteriolytic ability. Both the early reacting complement sequence and the alternate C3 activating pathway appeared to participate in killing as indicated by the roles of C2 and C3PA. Killing occurred, but with reduced efficiency, when either of the two substances was missing. However, serum lacking both C2 and C3PA could no longer kill Shigella. Killing also required the presence of C3, and presumably some of the later components of complement are subsequently involved.