Determination of Corynebacterium diphtheriae toxigenicity by a colorimetric tissue culture assay.

Chinese hamster ovary (CHO) cell cultures in microtiter wells are sensitive to growth inhibition and killing by picogram quantities of diphtheria toxin. In the absence of biologically active toxin, the CHO cell culture produces sufficient acidic metabolites to change the phenol red pH indicator from pink to yellow within 56 h. In the presence of 10 pg of toxin per well, growth inhibition can be observed microscopically within 24 h. Diphtheria toxin can be qualitatively assayed from culture supernatants of Corynebacterium diphtheriae or from beta-phage agar plaque plugs. The colorimetric CHO cell assay method for determining toxigenicity allows for the large-scale screening of either diphtheria toxigenicity or antitoxin titration of sera.