What does a reddish-pink color on MacConkey Agar indicate?

A reddish-pink color on MacConkey Agar indicates that lactose fermenting bacteria are present and are producing acid as a byproduct of fermentation. MacConkey Agar is both a selective and differential medium used to isolate and differentiate Gram-negative bacteria, particularly enteric bacteria.

In this medium, lactose serves as the primary fermentable carbohydrate, and the pH indicator (usually neutral red) is used to signal the production of acid from lactose fermentation. When lactose fermenters metabolize lactose, they produce acid, which lowers the pH of the medium. The change in pH results in the neutral red indicator undergoing a color change, leading to the reddish-pink coloration observed on the agar.

This distinctive color change helps in distinguishing lactose fermenters, such as Escherichia coli and Klebsiella pneumoniae, from non-lactose fermenters which typically do not produce acid and will appear colorless or transparent on the agar. Thus, seeing a reddish-pink color confirms that the organism is capable of fermenting lactose effectively and is indicative of a successful metabolic process.