Volutin granules

Volutin granules are an intracytoplasmic 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 display the metachromatic effect, appearing red when stained with methylene blue. 
They can also be found in the cytoplasm of "Saccharomyces", a genus of ascomycete fungi.
These may be of various types, the chief of which are volutin, polysaccharide, lipid and crystal. They are characteristic for different species and depend on the age and condition of the culture. Volutin granules (metachromatic or Babes-Ernst granules) are highly refractive, strongly basophilic bodies consisting of polymetaphosphate. They appear reddish when stained with polychrome methylene blue or toluidine blue (metachromasia). Special staining techniques such as Albert's or Neisser's demonstrate the granules more clearly. Volutin granules are characteristically present in diphtheria bacilli. Their function is uncertain. They have been considered to represent a reserve of energy and phosphate for cell metabolism but they are most frequent in cells grown under conditions of nutritional deficiency and tend to disappear when the deficient nutrients are supplied. Polysaccharide granules may be demonstrated by staining with iodine, and lipid inclusions with fat soluble dyes such as Sudan black. They appear to be storage products. Vacuoles are fluid-containing cavities separated from the cytoplasm by a membrane. Their function and significance are uncertain.