Ace the Cengage Microbiology Challenge 2026 – Dive into Your Discovery Adventure!

Selective media inhibit some microbes to let others grow, while differential media reveal differences between organisms that do grow. MacConkey agar does both: it uses bile salts and crystal violet to suppress Gram-positive bacteria, so Gram-negative enteric bacteria can thrive. It also contains lactose and a pH indicator, so lactose-fermenting organisms lower the pH and appear pink/red, while nonfermenters stay colorless. This combination makes MacConkey agar a classic example of a medium that is both selective and differential, allowing isolation of Gram-negative enterics and differentiation based on lactose fermentation.