Which organism is most likely to grow on mannitol salt agar?

Which organism is most likely to grow on mannitol salt agar?

• A. Streptococcus pyogenes
• B. Bacillus subtilis
• C. Clostridium perfringens
• D. Staphylococcus aureus

Answer: (D)

Mannitol Salt Agar is both selective and differential: the high salt concentration favors growth of Staphylococcus species, while the mannitol and pH indicator allow differentiation based on mannitol fermentation. If an organism ferments mannitol, the medium turns yellow due to acid production; if not, the medium stays red. Staphylococcus aureus is the classic organism that not only tolerates the high salt but also ferments mannitol, producing the yellow coloration that signals a positive result. The other organisms either do not grow well in such salty conditions or do not ferment mannitol on this medium, so they would not produce the characteristic yellow change. Thus, Staphylococcus aureus is the most likely to grow on and be identified by Mannitol Salt Agar.

Mannitol Salt Agar is both selective and differential: the high salt concentration favors growth of Staphylococcus species, while the mannitol and pH indicator allow differentiation based on mannitol fermentation. If an organism ferments mannitol, the medium turns yellow due to acid production; if not, the medium stays red.

Staphylococcus aureus is the classic organism that not only tolerates the high salt but also ferments mannitol, producing the yellow coloration that signals a positive result. The other organisms either do not grow well in such salty conditions or do not ferment mannitol on this medium, so they would not produce the characteristic yellow change. Thus, Staphylococcus aureus is the most likely to grow on and be identified by Mannitol Salt Agar.