What selective feature does MSA provide, and what indicates a positive mannitol fermentation?

What selective feature does MSA provide, and what indicates a positive mannitol fermentation?

• A. It inhibits Gram-positive bacteria and selects Gram-negative.
• B. Low salt selects streptococci; red colonies indicate fermentation.
• C. High salt selects staphylococci; yellow colonies indicate mannitol fermentation.
• D. Growth indicates negative mannitol fermentation

Answer: (C)

MSA is both selective and differential. Its high salt concentration creates an environment that allows Staphylococcus species to grow while inhibiting many other bacteria. For differentiation, the medium contains mannitol and a pH indicator. If an organism ferments mannitol, acid is produced, lowering the pH and turning the colony and surrounding medium yellow. So yellow colonies indicate mannitol fermentation. If fermentation doesn’t occur, the medium around the colony remains red. This is why the best description is that high salt selects staphylococci and yellow colonies indicate mannitol fermentation.

MSA is both selective and differential. Its high salt concentration creates an environment that allows Staphylococcus species to grow while inhibiting many other bacteria. For differentiation, the medium contains mannitol and a pH indicator. If an organism ferments mannitol, acid is produced, lowering the pH and turning the colony and surrounding medium yellow. So yellow colonies indicate mannitol fermentation. If fermentation doesn’t occur, the medium around the colony remains red. This is why the best description is that high salt selects staphylococci and yellow colonies indicate mannitol fermentation.