A Petri dish culture plate containing Tinsdale medium, inoculated with a mixed culture of Corynebacterium diphtheriae bacteria, which grew colonies that displayed the morphologically-expected dark-brown to black coloration of this Gram-positive bacterium. Tinsdale agar plate growth gave rise to black colonies with a brown halo, typical of cysteinase-producing C. diphtheriae, C. ulcerans, or C. pseudotuberculosis. Containing the hydrogen sulfide indicators, L-cysteine and sodium thiosulfate, Tinsdale medium is used when isolating and identifying C. diphtheria bacteria.