Introduction
MacConkey Agar is a solid medium recommended for use in qualitative procedures for the isolation of gram-negative bacilli and for the differentiation of those organisms on the basis of lactose fermentation.
The peptones in this medium provide nitrogenous nutrients and amino acids. Lactose is added as a carbon source for energy. Sodium chloride maintains osmotic equilibrium. This medium is selective due to the presence of crystal violet and bile salts, which are inhibitory to most gram-positive organisms. Differentiation of microorganisms is accomplished by the combination of lactose and the neutral red indicator. Coliform organisms, which ferment lactose, usually develop into red colonies surrounded by a zone of precipitated bile. Lactose nonfermenters such as Salmonella and Shigella develop into colorless, transparent colonies. The swarming of Proteus is inhibited on this medium; however, occasional strains will swarm to some extent.
Interpretation
Lactose-fermenting organisms grow as pink to brick-red colonies with or without a zone of precipitated bile. Lactose-nonfermenting organisms grow as colorless or clean colonies
Cat# BP-014
