Phylogenetic Aspects of Staphylococcal Protein A‐reactive Serum Globulins in Birds and Mammals

Serum samples from 300 individual birds representing 72 species from 15 of 28 living orders were tested for protein A reactivity in gel diffusion experiments. Only two species, Rhea americana (Greater Rhea) and Pterocnemia pennata (Lesser Rhea), showed positive reactions, whereas serum samples from the remaining 70 species tested were negative. Protein A reactivity in the two Rheiformes species was identified in a slowly migrating serum globulin of unknown function with a molecular weight of 185 000. Absorption of serum from Rhea americana with staphylococci containing protein A almost completely eliminated the slow globulin. In Pterocnemia pennata similar absorption experiments reduced the amount of the corresponding globulin by 40–50 per cent. Two mammalian sera, the wooly opossum and the wallaby, not tested previously were also analyzed. Both species showed marked protein A reactivity. The only negative mammalian serum found earlier, the American opossum, was also reexamined. Concentrated globulin fractions showed a quantitatively small but definite reactivity in tests for protein A reactivity. Thus, all 65 mammalian species tested contain gamma globulins reactive with protein A presumably via similar Fc structures of immunoglobulin G with a common evolutionary origin.