A protein antigen characteristic of Branhamella catarrhalis. Serological identification of the genus.

Precipitation patterns of sonicated, acid-extracted and other extracts from Branhamella catarrhalis were examined by double diffusion-in-gel technique, using antiserum to B. catarrhalis. Acid extract gave rise to 4 distinct precipitates. One of these lines was further studied. The bacterial component responsible for this line was trypsin-sensitive, indicating that it was a protein. It was anodally localized by crossed immunoelectrophoresis. By absorption of antiserum with whole bacteria, the precipitating capacity of the serum was diminished, suggesting that the protein antigen (P-antigen) was exposed on the bacterial surface. F(ab')22-fragments of IgG from antiserum, but not from normal rabbit serum, precipitated the P-antigen, indicating that it was a true antigen-antibody reaction. It was possible to make an IgG preparation monospecific for the P-antigen, by absorbing antiserum with trypsinized bacterial extract. 31 strains of B. catarrhalis, 9 strains of N. gonorrhoeae, 10 strains of N. meningitis, 12 other Neisseria spp. and 2 strains of H. influenzae were investigated for presence of cros-reacting surface antigens, using IgG monospecific for the P-antigen and 125I-labelled protein A from Staphylococcus aureus. After antibody exposure, all 31 strains of B. catarrhalis showed abundant uptake of protein A. No significant uptake was detected on any other investigated strain. Hence, the P-antigen appears to be characteristic of B. catarrhalis. The possibility of a serological identification of the species is introduced. Precipitating antibodies against the P-antigen were demonstrated in 69% of normal human sera.