For the isolation, enumeration and enrichment of Enterobacteriaceae. MacCONKEY AGARis used for detection and isolation of Gram-negative organisms fromwater4,pharmaceutical1,2,3and industrial5sources. MacConkey Agar conforms to harmonized USP/EU/JP1,2,3. Pancreatic digest of Gelatin is the nitrogen and vitamin sources in MacConkey Agar. Lactose Monohydrate is the fermentable carbohydratewith Neutral red serving as the pH indicator.Sodium chloride maintains the osmotic balance. Agar is the solidifying agent. Bile salts mixture and Crystal violet are the selective agents, inhibiting Gram-positive cocci and allowing Gram-negative organisms to grow. Lactose fermenting strains 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 strains, such as Shigella and Salmonella are colourless and transparent and typically do not alter appearanceof the medium. Inoculate MacConkey Agar (TMH 110) medium with a small number (not more than 100 cfu) of the appropriate microorganism and incubate at35±2°C for 18-24 hours. The growth of colonies indicates the possible presence of Escherichia coliand it shows red colour colonies.