What is the primary function of MAC (MacConkey) agar in microbiology?

The primary function of MacConkey (MAC) agar is to differentiate lactose fermenters from non-fermenters among Gram-negative bacteria. This selective and differential medium contains lactose, bile salts, neutral red, and crystal violet. The presence of lactose allows for the categorization of bacteria based on their ability to ferment lactose.

When lactose is fermented by certain bacteria, acid is produced, lowering the pH and resulting in a color change in the medium due to the neutral red indicator. This color change allows for easy identification of lactose fermenters, which will produce pink colonies on the agar, while non-fermenters will produce colorless or pale colonies. Thus, MAC agar is particularly useful in clinical laboratories for the rapid identification of enteric Gram-negative pathogens.

In contrast, isolating Gram-positive bacteria, culturing fungi, or identifying viral pathogens is not the primary role of MAC agar. The medium is specifically designed to target Gram-negative bacteria, and its selective properties inhibit the growth of Gram-positive organisms, while its formulation does not support the growth of fungi or facilitate the identification of viruses.