On MSA, what does a yellow colony indicate?

On MSA, what does a yellow colony indicate?

• A. Mannitol fermentation by that colony.
• B. No fermentation.
• C. Lactose fermentation.
• D. Glucose fermentation.

Answer: (A)

On Mannitol Salt Agar, a yellow colony means the organism ferments mannitol. Fermentation produces acid, lowering the surrounding pH and turning the medium’s phenol red indicator from red to yellow. This is the specific signal that distinguishes mannitol fermenters (like many Staphylococcus aureus strains) from nonfermenters. Lactose and glucose fermentation aren’t tested on MSA, so they wouldn’t cause a yellow color here, and no fermentation would keep the medium red instead of turning yellow.

On Mannitol Salt Agar, a yellow colony means the organism ferments mannitol. Fermentation produces acid, lowering the surrounding pH and turning the medium’s phenol red indicator from red to yellow. This is the specific signal that distinguishes mannitol fermenters (like many Staphylococcus aureus strains) from nonfermenters. Lactose and glucose fermentation aren’t tested on MSA, so they wouldn’t cause a yellow color here, and no fermentation would keep the medium red instead of turning yellow.