MacConkey agar is selective and differential:

* Selective: It inhibits the growth of Gram-positive bacteria by containing bile salts and crystal violet. These substances disrupt the cell walls of Gram-positive bacteria, making it difficult for them to survive.

* Differential: It differentiates between lactose fermenters and non-lactose fermenters. It contains lactose as a sugar source and a pH indicator, neutral red.

Here's how it works:

1. Lactose fermentation: Bacteria that can ferment lactose produce acids, which lower the pH of the medium.

2. pH indicator: Neutral red turns red in acidic conditions. Therefore, lactose-fermenting bacteria will produce red colonies on MacConkey agar.

3. Gram-positive bacteria: Most Gram-positive bacteria cannot ferment lactose and won't produce acids, resulting in colorless colonies. However, since they are inhibited by the selective components, they typically don't even form colonies.

In summary:

MacConkey agar is designed to isolate and differentiate Gram-negative bacteria based on their lactose fermentation abilities. It achieves this by inhibiting the growth of Gram-positive bacteria with its selective components.