Question

Consider a culture medium on which only gram- positive organisms such as Staphylococcus aureus colonies can grow due to an elevated NaCl level. A yellow halo surrounds the growth, indicating the bacterium fermented a sugar in the medium, decreasing the pH as a result and changing the color of a pH indicator chemical. This type of medium would be referred to as a(n)

1) selective and differential medium.
2) enrichment culture.
3) differential and enrichment culture.
4) differential medium.
5) selective medium.

Answer 1

Answer: It is a selective and differential medium.

Explanation: Selective medium is used to select the growth of certain bacteria whilst inhibiting the growth of another by preparing a medium with ingredients that will only select a specific growth of intended microbe. When closely related bacteria had been selected, they can further be differentiated by the way they utilize the medium. In this case staphylococcus aureus was first selected using sodium chloride with which only this bacteria can tolerate i.e halophilic (strives in salty environment). It was also further differentiated by its ability to ferment sugars changing the colour of the pH indicator chemical.