Question

Mannitol salt agar is selective because it:

• A. contains phenol red
• B. has high salt concentration
• C. supports fastidious organisms
• D. differentiates mannitol fermenters

Hint:

Remember:
• High salt \(\rightarrow\) Selective property
• Phenol red + mannitol \(\rightarrow\) Differential property \[ \textit{Staphylococcus aureus} \Rightarrow \text{Yellow colonies on MSA} \] because it ferments mannitol.

Answer 1

The Correct Option is B

Concept: Mannitol Salt Agar (MSA) is both:
• A selective medium
• A differential medium It is selective because of its: \[ \text{High salt concentration } (7.5\%~NaCl) \] The high salt content inhibits the growth of many bacteria while allowing salt-tolerant organisms, especially Staphylococcus species, to grow. It is differential because it contains mannitol and the indicator phenol red to distinguish mannitol fermenters.

Step 1: Understanding why Mannitol Salt Agar is selective. Selective media contain substances that permit growth of certain organisms while inhibiting others. In MSA: \[ 7.5\%~NaCl \] creates a high osmotic environment. Only salt-tolerant bacteria can survive under these conditions. Therefore: \[ \boxed{\text{Mannitol salt agar is selective because of high salt concentration}} \]

Step 2: Understanding the role of other components.
• Phenol red acts as a pH indicator.
• Mannitol fermentation produces acid: \[ \text{Acid production} \Rightarrow \text{Yellow color change} \] Thus, phenol red helps differentiate fermenters but is not responsible for selectivity.

Step 3: Eliminating the incorrect options.
• Option (A): Phenol red makes the medium differential, not selective.
• Option (C): MSA is not specifically designed for fastidious organisms.
• Option (D): Differentiation of mannitol fermenters is a differential property, not the selective property. Hence, the correct answer is option (B). \[ \boxed{\text{Selectivity of MSA} \Rightarrow \text{High salt concentration}} \]