is rich medium for microbiological culture medium MacConkey agar MacConkey medium An organic-rich, selective solid microbiological culture medium that contains peptones, lactose, bile salts, crystal violet, and a pH indicator (neutral red). The bile salts inhibit swarming in Proteus spp., and the crystal violet inhibits growth of some Gram-positive bacteria. Lactose fermentation lowers the pH, resulting in colonies that are red or pink in color, and causes the bile salts to precipitate. Non-lactose fermenters that can deaminate amino acids in the peptone release ammonia, which causes the pH to increase (and thus the colonies appear white). Macconkey agar MAC From MacConkey Agar (Becton-Dickinson Difco and BBL Manual of Microbiological Culture Media, 2nd edition): Intended Use MacConkey agars are slightly selective and differential plating media mainly used for the detection and isolation of gram-negative organisms from clinical,1-3 dairy,4 food,5-7 water,8 pharmaceutical, 9-11 cosmetic,6,7 and other industrial sources. MacConkey Agar is used for isolating and differentiating lactose-fermenting from lactose-nonfermenting gram-negative enteric bacilli. Principles of the Procedure Peptones are sources of nitrogen and other nutrients. Yeast extract is a source of trace elements, vitamins, amino acids and carbon. Lactose is a fermentable carbohydrate. When lactose is fermented, a local pH drop around the colony causes a color change in the pH indicator (neutral red) and bile precipitation. Bile salts, bile salts no. 3, oxgall and crystal violet are selective agents that inhibit growth of gram-positive organisms. Sodium chloride maintains osmotic balance in the medium. Magnesium sulfate is a source of divalent cations. Agar is the solidifying agent. Formulae Difco™ MacConkey Agar Approximate Formula* Per Liter Pancreatic Digest of Gelatin....................................... 17.0 g Peptones (meat and casein).......................................... 3.0 g Lactose...................................................................... 10.0 g Bile Salts No. 3............................................................. 1.5 g Sodium Chloride.......................................................... 5.0 g Agar.......................................................................... 13.5 g Neutral Red.................................................................. 0.03 g Crystal Violet............................................................... 1.0 mg pH 7.1 ± 0.2 Difco™ MacConkey Agar Base Consists of the same ingredients without the lactose. BBL™ MacConkey Agar Approximate Formula* Per Liter Pancreatic Digest of Gelatin....................................... 17.0 g Peptones (meat and casein).......................................... 3.0 g Lactose...................................................................... 10.0 g Bile Salts...................................................................... 1.5 g Sodium Chloride.......................................................... 5.0 g Agar.......................................................................... 13.5 g Neutral Red.................................................................. 0.03 g Crystal Violet............................................................... 1.0 mg pH 7.1 ± 0.2 Carrine Blank Maccongkey agar Mac-Conkey agar Escherichia coli K-12