Purple Agar is used for the study of carbohydrate fermentation reactions, particularly for the identification of gram-negative enteric bacteria when adding the desired carbohydrate. B-peptone and special HM peptone provide growing organisms with nitrogen and carbon compounds, long-chain amino acids and other essential nutrients, including nitrogen sources. Sodium chloride maintains the osmotic balance of the medium. Bromocresol violet is the indicator of pH, which turns yellow at acidic pH. Gas production is evident by the cleavage of the agar. The acid produced during carbohydrate fermentation causes bromocresol purple, the pH indicator, to turn yellow. If the carbohydrate is not used or fermented, the color of the medium remains unchanged or becomes more alkaline (darker purple) due to the decarboxylation of the amino acids present in the medium. It is recommended to add carbohydrates at a concentration of 1% to avoid possible reversion reactions, with the exception of glucose (dextrose).