Volutin granules are an intracytoplasmic (inside the cytoplasm of a cell) storage form of complexed inorganic polyphosphate, the production of which is used as one of the identifying criteria when attempting to isolate Corynebacterium diphtheriae on Löffler's medium. Polyphosphate granules are called metachromatic granules due to their displaying the metachromatic effect; they appear red when stained with methylene blue.
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Volutin granules Wikipedia(Text) CC BY-SA