What inhibitor is used in MacConkey medium?

MacConkey medium is a selective and differential culture medium primarily used to isolate gram-negative bacteria and differentiate lactose fermenters from non-fermenters. The key feature of this medium is its ability to inhibit the growth of gram-positive bacteria while allowing gram-negative bacteria to grow.

Crystal violet and bile salts serve as the inhibitors in MacConkey medium. Crystal violet inhibits the growth of some gram-positive organisms due to its ability to interfere with their cell wall synthesis. Bile salts further enhance this selective pressure by disrupting the membranes of gram-positive bacteria, which helps to ensure that the medium mainly promotes the growth of gram-negative bacteria.

The combination of these two inhibitors—crystal violet and bile salts—creates a more effective environment for isolating gram-negative organisms. Additionally, the medium contains lactose and a pH indicator (usually neutral red), which helps to visually differentiate lactose fermenters (which will produce acid and result in a color change) from non-fermenters.

Thus, the correct answer is that both crystal violet and bile salts are essential inhibitors used in MacConkey medium, working together to achieve its selectivity.