volutin

Volutin, also known as metachromatic granules or polyphosphate granules, are intracellular storage inclusions found in many bacteria. They consist mainly of inorganic polyphosphate (polyP), a linear polymer of phosphate residues, which aggregates into dense granules within the cytoplasm. Under light microscopy, volutin granules may be visualized by metachromatic staining methods that shift color; classic examples include Albert’s stain and Neisser stain, which render the granules as differently colored bodies compared with the surrounding cytoplasm.

Distribution and composition: Volutin granules are widespread among bacteria, including both Gram-positive and Gram-negative lineages. The granules may be associated with small basic proteins and metal ions; chemically they consist primarily of polyP complexed with cations such as magnesium. In many bacteria, the biosynthesis and turnover of polyP is regulated by polyphosphate kinases (PPK) and exopolyphosphatases (PPX).

Functions: The primary role is as a phosphate reserve that can be mobilized during phosphate limitation or energy metabolism; polyP can serve as a phosphate donor for biosynthesis and as a storage of energy equivalents in certain conditions. In some pathogenic bacteria, volutin granules or polyP metabolism may influence stress resistance or virulence traits, but the exact contributions vary by organism.

Clinical and taxonomic notes: Volutin staining can aid in the identification of certain bacteria in diagnostic microbiology, but it is not a universal diagnostic marker. Electron microscopy and molecular methods provide more definitive information about polyP metabolism.