Enzyme-linked immunosorbent assay for the quantitation of immunoglobulin G bound to Escherichia coli

Two strains of Escherichia coli were opsonized by incubation in heat-inactivated bovine blood serum and in whey. The opsonized bacteria were then immobilized by complexing with anti-bovine antibodies previously coated to walls of polystyrene tubes. The amount of bovine immunoglobulin (Ig) G in the immobilized complex was then determined by a direct enzyme-linked immunosorbent assay, using peroxidase as enzyme. Thus, a direct measurement of one class of opsonic substances on the surface of the organisms was determined. The sensitivity for measurement of IgG ranged from nanograms to micrograms. After incubation in blood serum, 500-fold more IgG was found on the surface of the serum-resistant strain. The amount of IgG absorbed from whey in each instance was much less than that absorbed from serum. The number of bound molecules per bacterium ranged between 2,000 and 2,000,000, depending on both the strain and the serum.