Mannitol salt agar (MSA) is a selective and differential medium. The selective agent and the differentiating property are respectively:

• A Bile salts (inhibit Gram-positives); lactose fermentation changing neutral red indicator
• B Potassium tellurite (inhibits Gram-negatives); black colony production by Staphylococcus
• C Gentamicin (inhibits Gram-negatives); mannitol fermentation showing gas production
• D 7.5% NaCl (inhibits most non-staphylococci); mannitol fermentation producing acid changes phenol red to yellow ✓

Explanation

Mannitol salt agar contains 7.5% NaCl, which inhibits most bacteria except halotolerant staphylococci, making it selective for staphylococci. It is also differential — it contains mannitol as the carbohydrate and phenol red as pH indicator. S. aureus ferments mannitol producing acid, turning the medium yellow around colonies, while coagulase-negative staphylococci generally do not ferment mannitol (medium remains red). MSA is used for isolation of S. aureus from wounds and nasal swabs. Bile salts in MacConkey agar inhibit Gram-positives; tellurite is in McLeod's medium.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.