Inclusion Bodies

A variety of inclusion bodies, granules of organic or inorganic material; for storage (carbon compounds, inorganic substances, and energy)

•      Reduce osmotic pressure by tying up molecules in particulate form

•      Some inclusion bodies lie free in the cytoplasm, not bounded by a membrane- polyphosphate granules, cyanophycin granules, and some glycogen granules

•      Other inclusion bodies are enclosed by a single-layered membrane (protein or lipid) - poly-β-hydroxybutyrate granules, some glycogen and sulfur granules, carboxysomes, and gas vacuoles

•      Quantity will vary with the nutritional status of the cell. For example, polyphosphate granules will be depleted in freshwater habitats that are phosphate limited.

 

Organic inclusion bodies 

• usually contain either glycogen or poly—hydroxybutyrate

•       Glycogen is a polymer of glucose units (long chains formed by (1→4) glycosidic bonds and branching chains connected to them by (1→6) glycosidic bonds)

•      PHB contains -hydroxybutyrate molecules joined by ester bonds 

 

•       Usually only one of these found in a species, purple photosynthetic bacteria have both

 

•      Poly--hydroxybutyrate readily stained with Sudan black for light microscopy and are clearly visible in the electron microscope

•      Glycogen can be seen only with the electron microscope. Stained with iodine 

•      Glycogen and PHB inclusion bodies are carbon storage reservoirs providing material for energy and biosynthesis. Many bacteria also store carbon as lipid droplets.

•      Cyanobacteria (blue green algae) - has two distinctive organic inclusion bodies

•       Cyanophycin granules -large polypeptides containing equal amounts of  arginine and aspartic acid. The granules are large enough to be visible in the light microscope and store extra nitrogen for the bacteria.

•       Carboxysomes are present in many cyanobacteria, nitrifying bacteria, and Thiobacilli. They serve as a reserve of the enzyme ribulose-1,5-bisphosphate carboxylase - may be a site of CO₂ fixation.

Gas vacuoles

•      In many cyanobacteria, purple and green photosynthetic bacteria, and a few other aquatic forms such as Halobacterium and Thiothrix

•      Help them to float at or near the surface; buoyancy

•      Gas vacuoles are aggregates of enormous numbers of small, hollow, cylindrical structures called gas vesicles ; composed entirely of a single small protein, to form a rigid enclosed cylinder that is hollow and impermeable to water but freely permeable to atmospheric gases

•       Bacteria with gas vacuoles can regulate their buoyancy to float at the depth necessary for proper light intensity, oxygen concentration, and nutrient levels

•       They descend by simply collapsing vesicles and float upward when new ones are constructed

Inorganic inclusion bodies

•      Polyphosphate granules or volutin granules – Polyphosphate is a linear polymer of orthophosphates joined by ester bonds.

•      Storage reservoirs for phosphate, an important component of cell constituents such as nucleic acids.

•      In some cells they act as an energy reserve, and energy source in reactions.

•      Also called metachromatic granules -show the metachromatic effect;  appear red or a different shade of blue when stained with the blue dyes methylene blue or toluidine blue

•      Sulfur granules - store sulfur temporarily

•      Eg., purple photosynthetic bacteria

•      Inorganic inclusion bodies can be used for purposes other than storage

Magnetosome (contain iron in the form of magnetite)-      Used by some bacteria (eg., Aquaspirillum magnetotacticum) to respond to the earth’s magnetic field