MacConkey agar

Definition

MacConkey agar is a selective and differential culture medium designed to isolate and differentiate enteric bacteria based on their ability to ferment lactose. It contains bile salts and crystal violet to inhibit the growth of Gram-positive organisms, and neutral red as a pH indicator.

5 Must Know Facts For Your Next Test

1. MacConkey agar is used to isolate Gram-negative bacteria while inhibiting Gram-positive organisms.
2. Lactose fermenters on MacConkey agar produce pink colonies due to acid production that lowers the pH, causing the neutral red indicator to turn red.
3. Non-lactose fermenters produce colorless or pale colonies since they do not lower the pH enough to change the indicator color.
4. Bile salts and crystal violet in MacConkey agar inhibit the growth of most Gram-positive bacteria.
5. It is commonly used in clinical laboratories for identifying members of the Enterobacteriaceae family.

Review Questions

• What is the purpose of bile salts and crystal violet in MacConkey agar?
• How can you differentiate between lactose fermenters and non-lactose fermenters on MacConkey agar?
• Why are Gram-positive bacteria typically inhibited on MacConkey agar?

Related terms

Selective Media:

Culture media designed to suppress the growth of unwanted microorganisms and support the growth of desired ones.

Differential Media:

Culture media that allows differentiation between different types of microorganisms based on their metabolic activities.

Enterobacteriaceae:

A large family of Gram-negative bacteria that includes many pathogens such as Salmonella, Escherichia coli, Yersinia pestis, Klebsiella, and Shigella.