Corynebacterium diphtheriae cystinase activity on Tinsdale agar. Colonies of C.diphtheriae are characteristically black (reduction of potassium tellurite by colonies of C.diphtheriae). The dark halo is due to the production of H2S from cystine, interacting with the tellurite salt. Other diphtheroids do not have this ability with the exception of C.ulcerans and C.pseudotuberculosis.
Some Staphylococcus spp. may also produce this characteristic halo.
Cultivation 48 hours, 37 °C in an aerobic atmosphere.