MacConkey agar – for the selective growth of Gram-negative bacteria and differentiation of lactose and non-lactose fermenting organisms. Lactose fermenting organisms form pink colonies on the surface of the agar. 500 g MacConkey Agar Use 50 grams with 1 liter of water. DOT Info: Non-regulated WARNING: This product can expose you to chemicals including Crystal Violet, which is known to the State of California to cause cancer. For more information go to http://www.P65Warnings.ca.gov.