Which medium is used for the primary isolation of Corynebacterium diphtheriae and differentiates colonies by appearing as black colonies with a brown halo?

• A. Tinsdale agar
• B. Mannitol salt agar
• C. Blood agar
• D. MacConkey agar

Answer: (A)

The main idea here is using a selective medium that highlights Corynebacterium diphtheriae by giving it a distinctive colony look. Tinsdale agar contains tellurite, which suppresses many other bacteria and lets diphtheriae grow. A hallmark is the dark, often black colony due to tellurite reduction, surrounded by a brown halo from tellurite deposition around the colony. This combination—selective growth plus the characteristic black colony with a brown halo—makes Tinsdale agar the best choice for primary isolation and preliminary identification of C. diphtheriae.

Other media listed aren’t selective for diphtheriae and don’t produce that tellurite-based appearance. Mannitol salt agar favors halotolerant Staphylococcus species; blood agar is non-selective and won’t differentiate diphtheriae visually; MacConkey agar targets Gram-negative enteric bacteria and similarly won’t reveal this distinctive colony morphology.