What does the MacConkey agar selective for?

MacConkey agar is a selective and differential culture medium specifically designed to isolate and differentiate gram-negative bacteria, particularly those that can ferment lactose. The agar contains bile salts and crystal violet, which inhibit the growth of most gram-positive organisms, thus allowing for the selective growth of gram-negative bacteria only.

In addition to its selective qualities, MacConkey agar differentiates lactose fermenters from non-fermenters. Lactose fermenters, such as Escherichia coli, produce acid as a byproduct of lactose metabolism, leading to a color change in the medium (often appearing pink). Non-fermenters do not produce acid and thus do not change the color of the medium.

This ability to both select for a specific group and differentiate within that group makes MacConkey agar a valuable tool in microbiology, particularly in clinical diagnostics and food safety.