Concentrated deposits of certain substances presented/located in the cytoplasm of certain bacteria are known as cytoplasmic granules or inclusion bodies. They serve as storage areas for nutrients, e.g. volutin granules are high energy reserves stored in the form of polymerized metaphosphate.
The number and nature of the inclusions vary depending on the bacterial species and the nutritional state of the organism’s environment. Common types of granules that are microscopically visible in certain bacteria stained with specific dyes are:
- Polysaccharide granules (glycogen): a storage form of glucose and
- polyphosphate granules: a storage form for inorganic phosphates (volutin granules, also known as metachromatic granules, are composed of polyphosphate)
- poly-β-hydroxybutyrate (PHB) granules: a reserve carbon and energy source.
- Elemental sulfur: Intracellular globules of elemental sulfur may accumulate in certain bacteria growing in environments rich in hydrogen sulfide.
Cytoplasmic granules/inclusion bodies stain characteristically with certain dyes;
- Volutin granules appear as “metachromatic” granules, stains intense reddish-purple color with methylene blue dye (instead of blue, as one would expect), and can be observed by light microscopy.
- Poly-β-hydroxybutyrate (PHB) granules can be stained with lipid-soluble dyes such as Nile blue.
- Glycogen can be stained brown with iodine.
Metachromatic granules are characteristics of Corynebacterium diphtheriae
