Susceptibility of Serratia marcescens to Human Serum: Antagonism of Serum Bactericidal Activity by IgG Immunoglobulins of Homologous Rabbit Anti-O Sera

Abstract

Absorption of fresh human serum with boiled or live homologous O-cells of Serratia marcescens in the cold (4 degrees C, 1 hour) completely abrogated killing activity against "delayed serum-sensitive" (DSS) strains of S. marcescens. In contrast, previously "promptly serum-sensitive" (PSS) strains of S. marcescens and the PSS control strain Escherichia coli C were killed kinetically in a slightly "delayed" fashion by antibody-depleted fresh human serum. Addition of 40 vol% of heat-inactivated homologous rabbit anti-O immune sera to 50 vol% of fresh human serum "blocked" bactericidal activity against DDS and PSS assay strains of S. marcescens. The "blocking" activity of rabbit anti-O immune sera was shown to reside in the crude gamma globulin fractions derived therefrom through precipitation with ammonium sulfate. Absorption of rabbit anti-O immune sera with boiled homologous O-cells failed to remove the "blocking" activity; the O-agglutination titers were reduced to nondetectable levels. However, dual absorptions of rabbit anti-O immune sera with killed cells of Staphylococcus aureus Cowan I (protein A) almost completely abolished "blocking" activity; the O-agglutinin titers of these immune sera were not reduced more than 4-fold. Immunoelectrophoresis and rocket electroimmuno assays. employing heavy chain-specific goat anti-rabbit IgG and IgM sera, respectively, established that dual protein A-absorption selectively removed rabbit IgG, but not rabbit IgM, immunoglobulins. It was concluded that rabbit immunoglobulins of the IgG class accounted for the observed in vitro antagonism of the bactericidal activity of fresh human serum against DSS and PSS strains of S. marcescens.