Differential media are specialized growth media used in microbiology to distinguish between different types of microorganisms based on their biochemical properties. These media contain specific nutrients and indicators that allow certain bacteria to produce visible changes, such as color changes or gas production, when they metabolize specific substrates. For example, MacConkey agar is a differential medium that helps identify Gram-negative bacteria. It contains lactose and a pH indicator, allowing lactose-fermenting bacteria to produce acid, resulting in a color change. This helps microbiologists differentiate between E. coli and other non-lactose fermenters, aiding in the identification of bacterial species.