MacConkey agar is a selective and differential culture medium used primarily for isolating and identifying Gram-negative bacteria. It contains bile salts and crystal violet, which inhibit the growth of Gram-positive bacteria, allowing for the growth of Enterobacteriaceae and other Gram-negative organisms. The medium is also designed to differentiate lactose fermenters from non-fermenters. Lactose fermenters, such as Escherichia coli, produce acid, leading to a color change in the agar, typically resulting in pink colonies. In contrast, non-fermenters, like Salmonella, remain colorless, making it easy to identify different bacterial species.