MacConkey Agar is one of the earliest selective and differential medium for cultivation of coliform organisms (1, 2). Subsequently MacConkey Agar and Broth has also been recommended for microbiological examination of foodstuffs (3) direct plating / inoculation of water samples (4) and examination of Milk and Dairy Products using Standard Methods (5). British pharmacopoeia (6) has recommended this medium for the subculture and identification of Escherichia coli. It has also been named as Agar Medium H. and is also recommended by USP/BP/EP/JP (6-9) for the name.
Pancreatic digest of gelatin and peptones (meat and casein) provide the essential nutrients, vitamins and nitrogenous factors required for growth of microorganisms. Lactose monohydrate is the fermentable source of carbohydrate. 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.
After enrichment of Escherichia coli in MacConkey Broth (DM1083B), it is then subcultured on MacConkey Agar. Gram-negative bacteria usually grow well on the medium and are differentiated by their ability to ferment lactose. Lactose fermenting bacteria grow as red or pink and may be surrounded by a zone of acid precipitated bile. The red colour is due to production of acid from lactose, absorption of neutral red and a subsequent colour change of the dye when the pH of medium falls below 6.8. Lactose non-fermenting bacteria such as Shigella and Salmonella are colourless and transparent and typically do not alter appearance of the medium. Yersinia enterocolitica may appear as small, non-lactose fermenting colonies after incubation at room temperature.



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