Question

Phenol red is another indicator commonly used to detect the presence of acid. This indicator is red above pH7 and yellow below pH of 7. What would be the appearance of E. coli (gram negative rod, lactose fermenter) and S. aureus (gram positive cocci, lactose fermenter) if neutral red in MacConkey agar was replaced with phenol red?

Answer 1

MacConkey agar is a selective agar for gram-negative rods that is used in laboratory to differentiate between lactose fermenting and lactose non-fermenting gram-negative rods. Lactose fermenting organisms produce acid hence, leading to production of an acidic environment and dropping the overall pH of the media. Neutral red and phenol red are pH indicators that are used for detection of such organisms on the basis of their capability to utilize sugar. The color of the colonies obtained in presence of these indicators is as follows:

1. Neutral Red: pH < 6.8 (Acidic): turns red pH > 6.8 (Basic): remains colorless
2. Phenol Red: pH < 6.8 (Acidic): turns yellow pH > 6.8 (Basic): turns red

E. coli: as it is a gram-negative rod and capable of utilizing lactose available in the agar hence, on replacing neutral red to phenol red pH indicator the color of the colonies will change to yellow from red.

However, as S. aureus is a gram-positive cocci hence, its growth is inhibited on the medium due to presence of bile salts and dye crystal violet. Therefore, its colonies will not appear on the medium.