Medium 7. MacConkey Broth

Principle And Interpretation

MacConkey Broth is a modification of MacConkey Medium (1). Childs and Allen (2) demonstrated the inhibitory effect of neutral red and therefore substituted it by the less inhibitory bromocresol purple dye. BCP is more sensitive in recording pH variation in the medium. Indian pharmacopoeia (3) has recommended this medium for the presumptive identification of coliforms from water and other materials of sanitary importance.

Gelatin peptone provides essential growth nutrients. Lactose is the fermentable carbohydrate. Dehydrated ox-bile inhibits gram-positive organisms. Bromo cresol purple is the pH indicator in the medium. Lactose fermenting organisms gives yellow colour to the medium due to drop in pH as a result of lactose fermentation. Lactose non-fermenting organisms like Salmonella and Shigella do not alter the appearance of the medium. Gas produced gets entrapped in inverted Durhams tube.

Transfer homogenate in Casein Soyabean Digest Broth (MM011) containing 1 gm or 1 ml of the preparation to be examined to 100 ml MacConkey Broth Incubation is carried at 43°-45°C for 24-48 hours. For further isolation subculture on MacConkey Agar (MM081). Growth of red generally non-mucoid colonies, sometimes surrounded by a reddish precipitation zone, indicates pressure of coliforms.