Which result would indicate that a bacterial strain is positive for mannitol fermentation when grown on mannitol salt agar?

• A. The medium turns yellow around the colonies.
• B. No growth is observed on the agar.
• C. Colonies produce a metallic green sheen.
• D. The medium remains red around the colonies.

Answer: (A)

Verified step by step guidance

1. Understand the purpose of mannitol salt agar (MSA): it is a selective and differential medium used to isolate and identify bacteria that can ferment mannitol, especially Staphylococcus species.
2. Recognize that MSA contains a pH indicator (phenol red) which changes color based on acid production from mannitol fermentation.
3. Recall that when bacteria ferment mannitol, they produce acidic byproducts that lower the pH of the medium, causing the phenol red indicator to change from red to yellow around the colonies.
4. Interpret the results: if the medium turns yellow around the colonies, it indicates mannitol fermentation (positive result); if the medium remains red, it indicates no fermentation (negative result).
5. Note that no growth on MSA suggests the bacteria are inhibited by the high salt concentration, and a metallic green sheen is characteristic of Eosin Methylene Blue agar, not MSA.