Question

What substance in MSA confers selectivity? Why? What substance makes MSA differential? Why? What purpose does phenol red serve in MSA? Why are the Nutrient agar plates used?

Answer 1

1. High salt concentration i.e. 7.5% sodium chloride makes MSA selective. Because it inhibits the growth of most of the organisms other than staphylococci.

2. The carbohydrate source mannitol which is used for fermentation and phenol red which is used as ph indicator makes MSA differential.

3. It is an indicator of pH. It changes to the phenol red to yellow if the organism can ferment the mannitol.

4. Nutrient agar plates are used for cultivation of microbes supporting growth of a wide range of bacteria as it contains many nutrients required for the bacterial growth.