Ferric iron and the antibacterial effects of horse 7S antibodies to Escherichia coli O111.

Previous work showed that the virulence of Escherichia coli O141 was related to its ability to secrete catechols capable of transporting iron from serum transferrin to the bacterial cell. E. coli O111 also produced similar compounds in synthetic medium but was unable to do so in serum. It was postulated that antibody interfered with the production of these substances by this strain. The present experiments show that horse antiserum to E. coli O111 can induce bacterial killing in foetal calf serum, which, in contrast to the bactericidal effect of normal rabbit serum, cannot be reversed by Fe3+. Five IgG subfractions of increasing electrophoretic mobility were isolated from the 7S fraction of the antiserum, all of which exerted a bactericidal effect on E. coli O111 in calf serum which could not be prevented by Fe3+. The bacteriostatic effect induced by the fractions in the presence of transferrin could, however, be reversed by Fe3+. IgG and IgT were isolated from normal serum but neither of these activated calf serum complement or induced bacteriostasis in the presence of transferrin. These results show that specific antibody is responsible for these antibacterial effects and point to new problems concerning the role of iron compounds in the antibacterial effects of normal and specific immune serum.