Comparison of direct and indirect immunoradiometric assays (IRMA) for Bacillus anthracis spores immobilised on...

A solid phase immunoradiometric assay (IRMA) is described in which Bacillus anthracis spores were heat fixed to the wells of glass multispot microscope slides. Assays for spores of B. anthracis Vollum and Sterne strains with 3H labels were evaluated in the direct and indirect versions. Neither single nor signal-to-noise characteristics of indirect assays were greatly improved by the use of immunopurified antibody (IPAB) or IgG anti-bacterial reagents rather than antiserum. However, the specificity of the direct and indirect assays for B. anthracis strains and B. cereus NCTC 8035 was altered by immunopurification of the anti-bacterial reagent. Although the signal-to-noise ratio was sometimes higher in indirect than in direct assays, signal values were usually no better. Evidence was produced that the overall ratio of the indirect : direct antibody molecules bound by preparations of B. anthracis spores rarely exceeded two but the antibody-molecular ratio for antigens on extracellular material in spore preparations was much higher than the ratio for antigens on the spores themselves.