Enhancement of Human Serum Bactericidal Activity by Specific IgM Antibodies of Normal and Anti-O Immune Rabbit Sera Against Pseudo-Serum-Resistant Strains of Serratia marcescens

Abstract

Abstract Fresh and heat-inactivated (56 °C, 30 min) normal (control) rabbit sera (at 40 vol%) were found to “enhance” the bactericidal activity of fresh human serum (50 vol%) against two „non-serum-sensitive“ (NSS) assay strains of Serratia marcescens (strains CDC O6:H3 and CDC O10:H8). Cells of these two strains were killed in a delayed fashion following addition of normal rabbit serum to human serum. However, strain PO16:H-proved as resistant as before. The crude gamma globulin fractions (at 10 vol%) derived from control rabbit sera “enhanced” human serum bactericidal activity against strains CDC O6:H3 and CDC O10: H8. Homologous anti-O rabbit immune serum, when added at 40 vol % to 50 vol% of fresh human serum, augmented the latter serum's bactericidal activity against S. marcescens strain CDC O6:H3, but not against the other two NSS assay strains. Dual absorptions of heat-inactivated normal rabbit serum with homologous O-cells of S. marcescens abolished the “enhancing” effect. In contrast, dual absorpitons of rabbit control sera with gently killed, protein A-carrying cells of Staphylococcus aureus strain Cowan I, a treatment known to selectively remove rabbit IgG but not IgM immunoglobulins, did not remove the “enhancing” effect against strains CDC O6:H3 and CDC O10:H8. Also, dual absorptions of homologous rabbit anti-O immune sera with protein A resulted in “enhancement” of human serum bactericidal activity against strains CDC O6:H3 and CDC O10:H8, a finding explicable by selective removal of competing (“blocking”) immune IgG antibodies by this treatment. It was concluded that O-antigen-specific, natural and immune antibodies from normal (control) and anti-O immune rabbit sera, respectively, of the IgM immunoglobulin class accounted for the observed “enhancement” of the bactericidal activity of fresh human serum against two of the three assay strains of S. marcescens. Since these two strains were merely apparently serum-resistant, in that their serum-resistance was overcome by the presence of adequate amounts of natural or immune specific IgM antibodies in this in vitro system, or by removal of competing (“blocking”) IgG immune antibodies from anti-O immune sera, it is proposed that these 2 strains be designated as “pseudo-serum-resistant” (PSR), in order to differentiate these from genuinely NSS strains of S. marcescens, such as strain PO16:H-.