General

MacConkey Agar is a weakly selective medium for the isolation of Enterobacteriaceae or specifically E. coli in pharmaceutical products, food, water, and other materials.

Enterobacteriaceae can be clearly identified even in the presence of Salmonella and Shigella.

The MacConkey Agar inhibits the growth of gram-positive bacteria by the addition of crystal violet and bile salts, whereas the growth of most gram- negative bacteria is supported. Bacteria degrading lactose to acids grow in pink to red colored colonies. Additionally E. coli and other acid forming bacteria will show a zone of precipitated bile salt around the colonies. Bacteria not degrading lactose will grow colorless.

The family of Enterobacteriaceae is defined by the growth on MacConkey Agar.

It is not possible to detect low numbers of Salmonella while the accompanying microbial flora is abundant.