Lactose-Fermenting Bacteria on MacConkey Agar

What color do lactose-fermenting bacteria such as Escherichia coli and Klebsiella appear on MacConkey agar?

Answer:

Lactose-fermenting bacteria such as Escherichia coli and Klebsiella appear pink or red on MacConkey agar.

MacConkey agar is a specialized growth medium used in microbiology to differentiate between lactose-fermenting and non-lactose fermenting bacteria. This agar contains lactose as well as pH indicators. When lactose-fermenting bacteria such as Escherichia coli and Klebsiella grow on MacConkey agar, they ferment the lactose. This fermentation process leads to the production of acidic byproducts, which lower the pH of the medium.

The lowering of the pH causes the pH indicators in the MacConkey agar to change color, usually turning pink or red. This change in color is a result of the acid produced by the lactose fermentation process. Therefore, when observing lactose-fermenting bacteria like Escherichia coli and Klebsiella on MacConkey agar, they will typically appear pink or red.