Which medium is both selective and differential?

Which medium is both selective and differential?

• A. Sheep blood agar
• B. Chocolate agar
• C. Mannitol salt agar
• D. Mueller-Hinton agar

Answer: (C)

The concept being tested is using a medium that is both selective and differential to isolate and identify a specific group of organisms. Mannitol salt agar achieves this by combining two features: a high salt concentration that inhibits most bacteria, so only Staphylococcus species typically grow, and a differential component (mannitol with a pH indicator). When an organism can ferment mannitol, it produces acid, lowering the pH and causing the indicator to change color (usually from red to yellow). This means Staphylococcus aureus, which ferments mannitol, will appear as yellow colonies or a yellow halo, while other non-fermenting staphylococci stay red/pink. The other media either lack selective pressure, are merely enriched or nonselective, and do not provide this combined isolation and differentiation.

The concept being tested is using a medium that is both selective and differential to isolate and identify a specific group of organisms. Mannitol salt agar achieves this by combining two features: a high salt concentration that inhibits most bacteria, so only Staphylococcus species typically grow, and a differential component (mannitol with a pH indicator). When an organism can ferment mannitol, it produces acid, lowering the pH and causing the indicator to change color (usually from red to yellow). This means Staphylococcus aureus, which ferments mannitol, will appear as yellow colonies or a yellow halo, while other non-fermenting staphylococci stay red/pink. The other media either lack selective pressure, are merely enriched or nonselective, and do not provide this combined isolation and differentiation.