Question

A scientist wants to culture Organism A, a bacterium that is
salt tolerant, penicillin resistant and turns bright purple in
the presence of chemical X. You are the culture media
consultant and must recommend ways that she might enrich
her nutrient agar to create a selective and a differential
media for Organism A. You recommend the following two
“recipes”. HINT This is an imaginary bacterium. You
are to design two imaginary media- do not suggest ones
such a MacConkey agar.
a. Differential media

b. Selective media

Answer 1

a. Differential media contains such ingredients that allow differentiating between two organisms mostly on the basis of the color they produced. In this case, our imaginary bacterium gives the color purple in the presence of Chemical X. So differential media can be made by adding chemical X to the nutrient agar. The imaginary bacterium will give bright purple colonies and if any other organism is present it will give white colonies. So culture can be isolated by picking the purple colonies.

b. Selective media contains such ingredients that allow the growth of one organism while inhibiting the growth of others. In this case, our imaginary bacterium is salt tolerant and penicillin-resistant, meaning it can grow in the presence of penicillin and salt, unlike most other bacteria. So selective media can be made by adding penicillin and high salt concentration to the nutrient agar. This will only allow the growth of our imaginary bacterium while inhibiting the growth of others thus acting as selective.

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