On a MAC plate, what does a pink coloration indicate?

When using a MacConkey (MAC) agar plate, the presence of a pink coloration indicates the growth of lactose fermenting colonies. MAC agar is specifically designed for the isolation of Gram-negative bacteria and differentiates lactose fermenters from non-lactose fermenters.

Lactose fermenting bacteria, such as Escherichia coli, can utilize lactose in the medium, producing acid as a byproduct. This acid lowers the pH of the medium, resulting in a pink color change around the colonies due to the alkaline indicator present in the agar. Therefore, the appearance of pink colonies signifies that the organism is capable of fermenting lactose.

In contrast, non-lactose fermenting bacteria do not produce acid and thus will not cause a color change, remaining colorless or producing beige colonies on the plate. This differentiation is a critical aspect of identifying enteric bacteria in clinical specimens.