Evidence for quantitative variability of bacterial opsonic requirements

We studied human serum opsonins by using combinations of heat inactivation and chelation to inhibit complement, adsorption to remove antibody, and trypan blue to inactivate the C3 receptor of human polymorphonuclear leukocytes. Streptococcus pneumoniae, serotype 25, required both complement and immunoglobulin for opsonization, even though that strain activated the alternative complement pathway. Both strains of Escherichia coli required antibody and complement, but varied in the degree of dependence on the C3 opsonin, since trypan blue moderately inhibited the killing of E. coli-1 and markedly inhibited the killing of E. coli-2. Serratia marcescens was opsonized in heat-inactivated serum (limited complement) or serum absorbed at 0 degrees C with S. marcescens (limited antibody), but depended on the C3 receptor in absorbed serum. S. marcescens activated the alternative pathway. Thus, opsonic requirements varied with the availability of opsonins. Requirements for bacterial opsonization vary with species and strains within species, perhaps reflecting quantitative relationships among alternative and classical pathway activation of C3, efficiency of adsorption of C3 or immunoglobulin G to bacterial surfaces, and efficiency of attachment of these ligands to polymorphonuclear leukocyte receptors. Furthermore, although not always sufficient for opsonization, the C3 opsonin (activated through either the classical or alternative pathway) appears necessary for effective phagocytosis and killing of all strains studied.