The specificity of agglutination reactions of Pseudomonas aeruginosa with O antisera

Polyagglutinable (PA) strains of Pseudomonas aeruginosa are agglutinated by more than one of the antisera prepared against antigenically unrelated O serogroups. They form c. 4% of strains--other than isolates from cystic-fibrosis units--submitted to us for typing. We were able to allocate 80% of PA strains to a single O serogroup by agglutination with typing sera that had been absorbed with the polyagglutinable strain SMC-247, or by co-agglutination tests with protein A-containing staphylococci coated with immunoglobulin from unabsorbed sera. Similar results were obtained by precipitation tests with crude bacterial extracts and unabsorbed sera, but these tests were less sensitive and less specific. Evidence is presented that PA antigen is a heat-stable cell constituent distinct from the O antigen. In rabbit antisera, anti-PA antibody is exclusively of the IgM class, but O antibody of both IgM and IgG classes is present.