Inclusion Bodies

Cytoplasm of the prokaryotic cells contains some organic and inorganic reserve bodies known as inclusion bodies. These inclusion bodies may or may not be surrounded by membranes. Cyanophycin granules, polyphosphate granules and some glycogen granules are a few examples of inclusion bodies which are not surrounded by membrane and lies free in the cytoplasm. Membrane surrounding the inclusion body is usually single layer and is around 2.0 to 4.0 nm thick. These membranes vary in composition; they can be protein in nature whereas other contains lipid. Examples of membrane enclosed inclusion bodies are carboxysomes, gas vacuoles, glycogen, sulfur granules and hydroxybutyrate granule. The main function of inclusion body is to store carbon compounds, inorganic substances and energy. The quantity of the inclusion bodies varies as per the nutritional requirement of the cell. For instance, phosphate granules are depleted in freshwater habitats that are phosphate limited. Inclusion bodies also ties up the molecules in particulate form thereby reducing the osmotic pressure. Some of the inclusion bodies are described below:

Glycogen

It is the polymer of glucose units. In this, glucose monomers are linked linearly by 1−> 4 glycosidic bonds and branching occurs by 1−> 6 glycosidic bonds. They are present in the form of granules in cytosol. When stained with iodine, they appear as reddish brown. They are basically the reservoirs of carbon, supplying materials for energy and biosynthesis. Purple photosynthetic bacteria contains glycogen.

Cyanophycin

Cyanophycin granules are large polypeptides containing equal amount of two amino acids i.e. arginine and aspartic acid. This amino acid polymer is not produced by ribosome and they are commonly found in cyanobacteria and a few heterotrophic bacteria. They can be viewed under light microscope because of their large size. Their main function is to store nitrogen for bacteria.

Carboxysomes

Carboxysomes are polyhedral inclusion bodies and are about 100 nm in diameter. They are mostly found in nitrifying bacteria, cyanobacteria and thiobacilli. They contains CO₂ fixation enzyme ribulose-1,5-bisphosphate carboxylase (RuBisCO) thereby acting as a site of CO₂ fixation.

Poly-hydroxybutyrate (PHB)

PHB are the polymers of the hydroxybutyrate molecules joined by ester bond between carboxyl group of one molecule and hydroxyl group of adjacent molecule. They are the reservoirs of carbon storage thereby supplying materials for energy and biosynthesis. They are commonly found in purple photosynthetic bacteria.

Gas vacuole

They are the aggregates of small, hollow and spindle shaped structure called gas vesicles. Gas vesicles are made up of proteins and they do not contain lipids. These hollow structures are impermeable to water but they are highly permeable to atmospheric gases. Gas vacuole regulates the buoyancy of bacteria and helps them to float at a depth necessary for adequate light intensity, nutrient level and oxygen concentration. They provide buoyancy by decreasing the overall cell density. These organic inclusion bodies are commonly found in purple and green photosynthetic bacteria, many cyanobacteria and in some aquatic forms like Halobacterium and Thiothrix.

Polyphosphate granules

Polyphosphate granules or volutin granules store inorganic phosphate. Orthophosphates are joined by ester bond to form linear polymer i.e. polyphosphate. These granules are basically the energy reservoir and acts as an energy source in many reactions. Volutin granule appears as red or different shades of blue when stained with blue basic dyes like methylene blue or toluidine blue. This is called metachromatic effect and hence granules are sometimes known as metachromatic granules.

Sulfur granules

Another type of inclusion body is sulfur granules. They are used to store sulfur. During photosynthesis in purple photosynthetic bacteria, hydrogen sulfide is used as an electron donor. The resulting sulfur is stored either in periplasmic space or in cytoplasmic globules.

Magnetosome

They are the membrane bound structure having many features similar to eukaryotic organelles. They are not used for storage purpose. But they are used like a compass needle by some bacteria to orient in the earth’s magnetic field. Magnetosome consist of magnetic mineral crystal (i.e. crystals of magnetite or greigite) surrounded by lipid bilayer. They are found in magnetotactic bacteria. These bacteria are motile and mostly aquatic.

Note: Glycogen, cyanophycin, carboxysomes, PHB and gas vacuole are organic inclusion bodies whereas polyphosphate granules, sulfur granules and magnetosome are inorganic inclusion bodies.