Corynebacterium diphtheriae on Tinsdale agar. Tellurite reduction and cystinase activity . 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.
(Organisms other than corynebacteria, such as staphylococci and streptococci, can reduce tellurite and produce black colonies.)
Cultivation 24 hours, 37 °C in an aerobic atmosphere.