What is the primary purpose of mannitol salt agar in microbiology labs?

• A. Differentiates Gram-positives
• B. Selects for halophiles and differentiates mannitol fermentation by Staphylococcus aureus
• C. Identifies lactose fermenters
• D. Selects for Gram-negatives

Answer: (B)

Mannitol Salt Agar is a selective and differential medium. The high salt concentration inhibits most bacteria except halotolerant Gram-positive cocci, especially Staphylococcus species, allowing these organisms to grow. It differentiates based on mannitol fermentation: Staphylococcus aureus ferments mannitol, producing acid that lowers the pH and turns the medium yellow around the colonies, while many other Staphylococcus species do not ferment mannitol and remain red. This combination makes the medium useful for selecting for halotolerant staph and distinguishing S. aureus from other staphylococci. It is not used to identify lactose fermenters or to select for Gram-negatives; those traits are assessed with other media.