Physiochemical consequences of opsonization of Salmonella typhimurium with hyperimmune IgG and complement

Partition in an aqueous, two-polymer phase system containing dextran and polyethylene glycol was employed to investigate the physico-chemical changes inflicted upon the cell surface of a smooth strain of Salmonella typhimurium by the binding of antibody IgG and complement. The minimum antibody concentration for increased phagocytosis in vitro was approximately the same as that for a significant change in two-phase partition, ca 8000 mol/bacterium, whereas a lower concentration, less than 4000 mol/bacterium, was sufficient to increase clearance in vivo. After pepsin digestion of IgG, larger quantities, ca 35,000 mol/bacterium, was required for opsonization and to influence two-phase partition. Addition of normal rabbit or guinea-pig serum to bacteria sensitized with a low concentration of antibody IgG conspicuously enhanced phagocytosis and affinity for the dextran-rich phase. The results show that binding of 8000 IgG antibody molecules or more to smooth S. typhimurium generates physicochemical changes of the bacterial surface which from studies on S leads to R mutations are known to correlate with hydrophobicity, negative charge and phagocytosis. Such results support the view that one important function of IgG antibody and complement is to decrease the hydrophilic properties of the bacteria which is thought to be a prerequisite for phagocytosis.