R2A agar is used for the enumeration and cultivation of bacteria from drinking water. R2A agar developed by Reasoner and Geldreich is a low nutrient medium that can used in pour plate, spread plate, and membrane filtration methods for heterotrophic plate counts. In combination with a lower incubation temperature and longer incubation period R2A agar stimulates the growth of stressed and chlorine-tolerant bacteria. Traditionally nutritionally rich media have been used for this purpose but these media support the growth of fast-growing bacteria and may suppress slow growing or stressed bacteria found in treated water.
Enzymatic digest of casein, proteose peptone, acid hydrolysate of casein and yeast extract provide the required nitrogen, carbon and vitamins. Glucose is a fermentable carbohydrate. Dipotassium hydrogen phosphate is a buffering agent. Magnesium sulphate is a source of divalent cations and sulphate. Starch and sodium pyruvate aid in the recovery of stressed organisms.