Purple Broth Base is used for the preparation of liquid fermentation media. Bacteria that can ferment the carbohydrate source turn the medium yellow. Peptone provides nitrogenous sources for bacterial growth. A fermentable carbohydrate must be added. Sodium chloride maintains the osmotic balance. Bromocresol Purple is the pH indicator, which turns yellow at acidic pH. If the carbohydrate is not utilized or fermented, the color of the medium remains unchanged or becomes darker purple due to increased alkalinization upon decarboxylation of the amino acids present in the medium. Gas production is evident by its collection in Durham's tube.