MacConkey Agar, Modified, 500 g

MacConkey Agar is the earliest selective and differential medium for cultivation of enteric microorganisms from a variety of clinical specimens (1, 2). The original medium contains protein, bile salts, sodium chloride and two dyes. The selective action of this medium is attributed to crystal violet and bile salts, which are inhibitory to most species of gram-positive bacteria. Klebsiella species are often associated with coliforms in water supply distribution systems and are present as a major component in industrial wastes of paper mill, textile and other industries. Thom (1970) (3) developed a medium based on MacConkey Agar in which lactose is replaced by inositol with the addition of 100µg of carbenicillin per ml. Bagley and Seidler (1978) (4) devised a similar medium with only 50µg of carbenicillin per ml. In the modified MacConkey agar medium (M051), inositol is incorporated in place of lactose while added carbenicillin makes the medium selective for Klebsiella species. Further, this method reduces the necessity for biochemical testing of pure strains; however, preliminary verification of differentiated colonies is recommended. Peptones are sources of nitrogen and other nutrients. inositol is a fermentable carbohydrate, bile salts and crystal violet are selective agents that inhibit growth of gram-positive organisms. 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. MacConkey, 1900, The Lancet, ii:20. 2. MacConkey, 1905, J. Hyg., 5:333. 3. Thom B. T., 1970, Lancet 2:1033 4. Bagley S. T. , Seidler R. J. , Tablbot H. W. and Morrow J. C., 1978, Appl. Environ. Microbiol., 36:178-185