MacConkey Agar is a selective and differential culture medium used primarily for isolating and identifying Gram-negative bacteria, particularly those in the Enterobacteriaceae family. It contains bile salts and crystal violet, which inhibit the growth of Gram-positive bacteria, allowing for the selective growth of Gram-negative organisms. The medium also contains lactose and a pH indicator, which helps differentiate between lactose fermenters and non-fermenters. When lactose is fermented, acid is produced, leading to a color change in the agar, typically resulting in pink colonies for lactose fermenters and colorless colonies for non-fermenters.