Phenol Red Dextrose Agar is similar Dextrose Agar with the addition of Phenol red as a pH indicator and is recommended to determine the ability of various organisms to ferment dextrose. Being a solid medium, it has the advantage of allowing fermentation reactions under aerobic and anaerobic reactions. Peptone mixture provides nitrogen, vitamins, minerals, and amino acids essential for growth, and allows the abundant growth of a wide variety of fastidious microorganisms. Sodium chloride supplies essential electrolytes for transport and osmotic balance. Dextrose is the fermentable carbohydrate providing carbon and energy. Phenol red is the pH indicator and bacteriological agar is the solidifying agent. Phenol Red Dextrose Agar is an excellent substrate for streptococci, as well as for other less fastidious bacteria. A yellow colour indicates fermentation, as the acid production reacts with the Phenol red pH indicator. The formation of gas causes bubbles to appear at the base of the medium, which can lead to agar fragmentation. A control of Phenol Red Agar without carbohydrates should be used to control false positives.
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