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Cell inclusion Bodies
Bacterial cell inclusion bodies are otherwise known as granules. Electron microscopic observation reported the presence of both organic and inorganic granules inside the bacterial cell. These granules function as energy storage and these are also involved in reducing osmotic pressure. Some of these granules are found freely moving in the cytoplasm (polyphosphate, glycogen, etc.) while some other granules are found inside of an enclosed cell (poly beta hydroxyburate). These granules are broadly classified into two groups on the basis of the chemical properties.
Image Source: Akira Kitamura
Glycogen is a long chain of glucose. The chain is formed by alpha 1-4 glycosidic bond and the branching chains are connected to these chains by alpha 1-6 glycosidic bond. Glycogen granules are found in the cytoplasm and visible under the electron microscope. However, a large number of glycogen granules are visible in the light microscope by iodine staining (reddish-brown appearance). Glycogen functions as carbon storage which provides energy for biosynthesis.
Poly Beta hydroxyburate
Beta hydroxyburate chains are formed by joining with ester bond between the carbonyl and hydroxyl group. These granules are visible under a light microscope by staining with Sudan black. These appear as empty holes under the electron microscope. Poly beta hydroxyburate granules are also used as carbon sources of the bacteria.
These granules are present in cyanobacteria. These are large polypeptides which are easily visible under a light microscope. These granules function as nitrogen storage.
These are found in cyanobacteria and carbon dioxide fixing bacteria. Carboxysomes are characterized by polyhedral structure. These granules contain an important enzyme called ribulose 1-5 biphosphate carboxylase (Rubisco) which plays an important role in carbon dioxide fixation.
Several aquatic prokaryotes have one unusual function called buoyancy. Different photosynthetic bacteria, halobacterium contain gas vacuoles. These are generally small hollow cylindrical structure which facilitates air permeability. The cell wall of gas vacuole contains single small proteins which involved information of a rigid cylinder.
These are also known as volutin granules and functions as phosphate storage. These are linear polymer of orthophosphate connected by an ester bond. Phosphate is an important component of nucleic acid. These are also known as metachromatic granules. These appear red or different shed of the blue by staining with methylene blue or toluidine blue.
Several prokaryotes contain sulfur granules which function as temporary sources of sulfur. For example, photosynthetic bacteria use sulfide as photosynthetic electron donor and accumulate the resulting sulfur in either periplasmic space or in cytoplasmic granules.
This is a special type of inclusion body found in bacteria present in the magnetic field of the earth.
Prescott, Harley & Klein’s Microbiology, 7th Edition.