MacConkey Agar is recommended for isolation, identification and enumeration of Staphylococcus aureus and Faecal Streptococci.(1)MacConkey Agar is the earliest selective and differential medium for cultivation of enteric microorganisms from a variety of clinical specimens (2,3). Subsequently MacConkey Agar and Broth have been recommended for use in microbiological examination of foodstuffs (4) and for direct plating / inoculation of water samples for coliform counts (5). These media are also accepted by the Standard Methods for the Examination of Milk and Dairy Products (6) and pharmaceutical preparations (7). Original medium contains protein, bile salts, sodium chloride and two dyes. Gram-negative bacteria usually grow well on the medium and are differentiated by their ability to ferment lactose. Lactose fermenting strains grow as strains grow as red or pink and may be surrounded by a zone of acid precipitated bile. The red color is due to production 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 strains, such as Shigella and Salmonella are colorless 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.
Bacteria causing food poisoning are also isolated on MacConkey Agar, such as Staphylococcus aureus and Faecal Streptococci.
Storage and Shelf-life:
Store below 30°C in tightly closed container and the prepared medium at 2 - 8°C. Use before expiry date on the label.
References:
1. Bureau of Indian Standards IS :5887 (Part II)- 1976, reaffirm 1986.
2 MacConkey, 1905, J. Hyg., 5:333.
3. MacConkey, 1900, The Lancet, ii:20.
4. Speck M.(Ed), 1985, Compendium of methods for the Microbiological Examination of Foods, 2nd ed., APHA, Washington, D.C.
5. Greenberg A.E., Clesceri L.S. and Eaton A.D, (Eds), 1992, Standard methods for the Examination of Water and Wastewater,18th ed., APHA, Washington, D.C.
6. Marshall R. (Ed), 1992, Standard methods for the Examination of Dairy products, 16th ed., APHA, Washington, D.C.
7. The United States Pharmacopoeia XXI and the National Formulary, 16th ed., 1985, United States Pharmacopoeial Convention, Inc, Washington, D.C.