MacConkey agar was the first solid differential media to be formulated which was developed in the 20th century by Alfred Theodore MacConkey. MacConkey Agar is the earliest selective and differential medium for the cultivation of coliform organisms. It is used for the isolation and differentiation of non-fastidious gram-negative rods, particularly members of the family Enterobacteriaceae and the genus Pseudomonas.
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| Ingredients | Amount |
| Peptone (Pancreatic digest of gelatin) | 17 gm |
| Proteose peptone (meat and casein) | 3 gm |
| Lactose monohydrate | 10 gm |
| Bile salts | 1.5 gm |
| Sodium chloride | 5 gm |
| Neutral red | 0.03 gm |
| Crystal Violet | 0.001 gm |
| Agar | 13.5 gm |
Pancreatic digest of gelatin and peptones (meat and casein) provide the essential nutrients, vitamins, and nitrogenous factors required for the growth of microorganisms. Lactose monohydrate is the fermentable source of carbohydrates . The selective action of this medium is attributed to crystal violet and bile salts, which are inhibitory to most species of gram-positive bacteria. Sodium chloride maintains the osmotic balance in the medium.
Lactose fermenting strains grow red or pink. The red color is due to the production of acid from lactose, absorption of neutral red, and a subsequent color change of the dye when the pH of the medium falls below 6.8. Non-lactose fermenting bacteria such as Salmonella, Proteus, Pseudomonas aeruginosa, and Shigella cannot utilize lactose and will use peptone instead. This forms ammonia, which raises the pH of the agar, and leads to the formation of white or colorless colonies formed in the plate and the agar surrounding the bacteria remains relatively transparent. These colonies can also look golden to brown with dark centers.
Lactose can be replaced in the medium by other sugars and the replacement sugars can be used to test the abilities of gram-negative bacteria to ferment the replacement sugars. For example, sorbitol can be used as a replacement sugar in MacConkey agar to assist in the isolation and differentiation of enteropathogenic E. coli serotypes such as E. coli O157:H7 by the presence of white circular colonies that are non-sorbitol fermenting.
| Organisms | Growth results |
| Escherichia coli | Pink to rose-red colonies (may be surrounded by a zone of precipitated bile) |
| Enterobacter, Klebsiella | Mucoid, pink colonies |
| Proteus | Colorless colonies, swarming growth |
| Salmonella, Shigella | Colorless colonies, or sometimes medium color: orange to amber |
| Pseudomonas | Irregular, colorless to pink colonies |
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