Which is the characteristic colony description of Corynebacterium diphtheriae on Tinsdale agar?

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

Answer: (C)

On Tinsdale agar, Corynebacterium diphtheriae reduces the tellurite in the medium, producing dark, nearly black colonies. The reduction also creates a brown halo around the colonies as tellurite diffuses into the surrounding agar. This distinctive black colony with a brown halo is a classic presumptive clue for diphtheriae on this selective medium. Other colony appearances like liquefaction or pits are not characteristic reactions on Tinsdale for this organism, and opalescent colonies with a white precipitate don’t reflect the tellurite reduction pattern seen here.