A solid-phase immunoassay for detection of IgA1 protease activity on agar plates

An immunoassay was developed which is capable of detecting IgA1 protease positive microorganisms from biologically derived samples grown on agar plates. Activity was detected by overlaying plates with agar containing insolubilized IgA and capturing Fab fragments on nitrocellulose disks coated with anti-lambda light chain. Using cultures of human saliva, a comparison of this assay with a more traditional method involving electrophoresis and Western blotting of individual colony material revealed excellent agreement between the two methods. This assay should be useful in studying the distribution and ecology of organisms producing IgA1 protease, and in assessing its potential as a virulence factor.