ELISA has frequently been used during the last few years for detection of proteins. ELISA employs the same principles as the radioimmunoassay, however, in stead of a radioactive label an enzyme label is used. Systems of ELISA to be used are e.g. the competitive ELISA, the sandwich ELISA and the double sandwich ELISA. All these systems require at least one very pure component and since very specific antibodies are more easily obtainable in quantity than the antigen itself, we developed the sandwich ELISA for detecting enterotoxin (Notermans et al., 1978). For this, polystyrene tubes are coated with S. aureus enterotoxin antibodies. Following incubation with the toxin, the bound toxin is measured using S. aureus enterotoxin antibodies conjugated with horse radish peroxidase. The amount of bound enzyme is determined spectrophotometrically using a specific substrate for this enzyme.
Using this system Protein A, produced by S. aureus, may cause a high cross-reactivity due to its binding to the FC fragments of the IgG antibodies. Interference by Protein A is completely eliminated by using the F(ab')2 fragment of lgG. Furthermore, it is necessary to remove nonspecific components of the F(ab')2 by affinity chromatography.
ELISA experiments in which this purified F(ab')2 was used resulted in a highly specific detection of enterotoxin both in culture filtrates and in foods (Koper, Hagenaars and Notermans, 1979).