On Tinsdale agar, Corynebacterium diphtheriae colonies are described as which of the following?

• A. Liquefaction of the agar surrounding the colonies on the medium
• 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)

On this medium, the distinguishing finding for Corynebacterium diphtheriae is dark, almost black colonies with a brown halo around them. Tinsdale agar contains potassium tellurite, which many bacteria can’t tolerate. C. diphtheriae reduces tellurite to telluride, producing the black pigment in the colonies, and a surrounding brown halo forms from tellurite precipitate in the agar around the colony. Other described patterns like agar liquefaction, opalescent colonies with a precipitate, or pitting don’t match this characteristic appearance on Tinsdale agar.