In vitro assays for detection of diphtheria toxin

Subunit-specific antipeptide antibodies have been used to develop a dot blot immunoassay for detection of diphtheria toxin (DT) produced by toxigenic strains of Corynebacterium diphtheriae. A detection limit of 0.1-1.0 ng DT/ml was obtained. DT is cytotoxic to cultured Vero cells and cell viability if conveniently assessed using a tetrazolium dye, MTT. This has been used as the basis of a cytotoxicity assay for DT, which has a detection limit of 20 pg/ml of biologically active toxin. Sixty isolates of C. diphtheriae were assayed for toxigenicity. There was complete concordance between the Vero cell cytotoxicity assay and guinea pig intradermal assay in vivo, demonstrating the utility of the cytotoxicity assay.