On Tinsdale agar, colonies of Corynebacterium diphtheriae are characterized by which appearance?

• A. Liquefaction of the agar surrounding the colonies
• B. Opalescent colonies with a white precipitate in the surrounding agar
• C. Black colonies on the culture medium surrounded by brown halos
• D. Pitting of the agar medium surrounding the colonies

Answer: (C)

Corynebacterium diphtheriae on Tinsdale agar forms black colonies with a surrounding brown halo. This distinctive look comes from the medium’s tellurite component. The organism reduces potassium tellurite to elemental tellurium, giving a dark, almost black center to the colony, and the diffusion of reduced tellurium into the surrounding medium produces a brown halo around it. This combination—black central colonies with brown halos—is a hallmark pattern used to identify C. diphtheriae on Tinsdale agar. Other appearances, like agar liquefaction around colonies, opalescent colonies with a white precipitate, or pitting of the agar, do not reflect this tellurite-reduction pattern and are not characteristic of C. diphtheriae on this medium.