Andrade Peptone Water, Modified

Principle And Interpretation

Bacteria differ widely in their ability to metabolize carbohydrates and related compounds. Carbohydrate fermentation reactions aids in the differentiation and identification of various bacteria. Andrade Peptone Water, Modified is prepared in accordance with FDA BAM, 1998 for carbohydrate fermentation studies of particularly Enterobacteriaceae members (1). Desired carbohydrate is added to the medium, which is inoculated with the test organism. If the test organism metabolizes the added carbohydrate, acids are produced, thereby lowering the pH of the medium. This causes a subsequent colour change of the indicator, from colourless to pink to red. If the added carbohydrate is not metabolized, the medium remains pale tan to straw coloured. Gas produced during fermentation is collected in the Durhams tube. Peptone used in the medium is free from fermentable carbohydrates (2,3) and is also free from nitrates which may interfere with gas production. Beef extract is an additional source of nutrients. Andrade indicator is a solution of acid fuchsin which changes colour from pink to yellow under alkaline conditions and yellow to pink under acidic conditions (1). The medium is pink when hot but becomes straw coloured on cooling. Test carbohydrate solutions should be sterilized separately and added aseptically to the sterile media. The biochemical identification of organisms capable of growing in this medium is made by various sugar fermentation results (2,4,5). Use fresh cultures of organisms, which have been presumptively identified by Gram staining and colony morphology. Biochemical tests are required for final identification of the bacteria.