Serological activity of protein A of Staphylococcus aureus: the precipitinogen as an antigen for determining antibodies by the passive hemagglutination test.

Tanned sheep erythrocytes have been considered incapable of sensitization with the precipitinogen of protein A of Staphylococcus aureus, so that the activity of this antigen in serological reactions has so far been studied by means of the agar diffusion test (ADT) only. The precipitinogen of the protein A in this study was found to become attached to the tanned erythrocytes and to sensitize them for the passive hemagglutination test (PHT). It was determined that, in contrast to nonspecific reactivity between normal human serum and the precipitinogen in the ADT, the reaction in the PHT was of specific nature. Of seven species studied, all normal human, dog, and hog sera tested were positive in the PHT. However, the hemagglutinin titers of the sera of the two animal species by far exceeded those of the human sera. The data emphasized the usefulness of the highly sensitive PHT for assaying antibodies to protein A.