How Do Inclusion Bodies Support Cellular Function?
If someone questions what cell inclusions are or what are inclusion bodies, the answer to both will be the same. Also known as inclusion bodies, these elementary bodies are cytoplasmic inclusions or nuclear aggregates of stable substances like proteins. They freely suspend and float within the cytoplasmic matrix and can also be called cytoplasmic inclusions. Some inclusion bodies lie free in the cytoplasm whereas some are enclosed by a shell. They are usually sites of viral multiplication, thus in a bacterium or eukaryotic cell, therefore, are also referred to as viral inclusion bodies that consist of viral capsid proteins. Inclusion bodies can be an indication of certain diseases like Herpes, Parkinson’s disease, Measles, Rabies and Dementia.
Features of Inclusion Bodies
These act as reserve deposits.
Plenty of nutrients can be stored in them by the cells and utilized when there is deficiency in the environment.
Some of the inclusion bodies in bacteria are very common in a wide variety.
Cell inclusions are generally acidophilic.
These can also be present as crystalline aggregates of virions.
Represent degenerative changes produced by a viral infection.
These are made of virus antigens present at the site where synthesis of virus takes place.
These can be observed as pink structures under microscope when stained with gypsum or methylene blue dye.
Classification of Inclusion Bodies
Inclusion bodies are being classified into two types, namely: Organic Inclusion bodies and inorganic inclusion bodies.
The Organic Inclusion Bodies
Organic inclusion bodies usually contain either (1) glycogen granules or (2) poly-β-hydroxybutyrate (PHB)
Carbon storage reservoirs like glycogen and PHB inclusion bodies provide material for energy and biosynthesis. Glycogen is a polymer of glucose units made up of long chains connected by glycosidic links and branching chains linked by glycosidic bonds. Staining cells with an iodine solution turns them reddish-brown if they contain a lot of glycogen.
Poly-β-hydroxybutyrate (PHB) contains hydroxybutyrate molecules joined by ester bonds between adjacent molecules' carboxyl and hydroxyl groups. Beta hydroxy is the name given because of the portion of the OH group in comparison to the double-bonded Oxygen group. Since the group is repeated several times in the structure, thus it is known as Poly- β-hydroxybutyrate
Cyanobacteria is basically a group of bacteria that can photosynthesise and they have two distinctive organic inclusion bodies:
Large polypeptides with almost equal amounts of two types of amino acids arginine and aspartic acid. One is a basic amino acid, another being an acidic amino acid, which makes up the cyanophycin granules.
Carboxysomes are found in many cyanobacteria and other CO2-fixing bacteria, and they store additional nitrogen for the organism.
The gas vacuole is an amazing organic inclusion body in prokaryotic organisms. Some aquatic prokaryotes rely on it for buoyancy.
Prokaryotes that have gas vacuoles in them can regulate their buoyancy to float at the depth necessary for proper light intensity, oxygen concentration, and nutrient levels. They float upward after new vesicles are formed and down by collapsing vesicles.
Inorganic Inclusion Bodies
In prokaryotes, there are two types of inorganic inclusion bodies:
Granules of Polyphosphate
Many bacteria store phosphate in the form of polyphosphate granules or volutin granules. Polyphosphate is an ester-linked linear polymer of orthophosphates. As a result, volutin granules serve as phosphate storage reservoirs, a key component of cell elements like nucleic acids.
They serve as an energy store in some cells, and polyphosphate can be used as an energy source in processes. These granules are also known as metachromatic granules. They have a metachromatic effect when stained with the blue dyes methylene blue or toluidine blue, which causes them to appear reddish or different shades of the blue spectrum.
Sulfur Granules
Some prokaryotes use sulfur granules to store sulfur temporarily. Photosynthetic bacteria, for example, can use hydrogen sulfide as a photosynthetic electron donor and store the sulfur produced in the periplasmic space or specific cytoplasmic globules.
Metachromatic Inclusions - Some of the large cell inclusions that sometimes stain red with blue dyes like methylene blue are called metachromatic inclusions. These consist of granules that consist of starch and glycogen. In the presence of iodine, the glycogen granules appear reddish brown in color and the starch granules appear blue.
Lipid Inclusions - These appear in various species of Bacillus, Mycobacterium, Azotobacter, and other genera and act as lipid storage material. These can be observed when cells are dyed with fat soluble dyes like Sudan dyes.
Sulfur Granules - These derive energy by oxidizing sulfur and sulfur containing compounds. They may deposit sulfur granules in the cell that serve as an energy reserve.
Carboxysomes - These inclusions contain the enzyme ribulose 1, -5 diphosphate carboxylase. Bacteria use these for a source of carbon for carbon dioxide fixation during photosynthesis.
Magnetosomes - Some bacteria orient themselves within a magnetic field due to the presence of magnetosomes. Magnetosomes are intracellular inclusion bodies or particles of iron oxide mineral called magnetite (Fe3O4). A magnetosome is enclosed by a thin membrane composed of protein, phospholipid and glycoprotein. It imparts a magnetic dipole on a cell allowing itself to exhibit magnetotaxis which is the process of migrating along the earth’s magnetic field. It is found in many aquatic organisms.
Based on the location, either at the nucleus or cytoplasm or at both of these cell organelles, we can classify inclusion bodies in the following categories:
Intranuclear cell inclusions.
Infection inclusion bodies.
Intracytoplasmic inclusion bodies.
Physiological inclusion bodies.
Inclusion bodies can be present in a bacterium or eukaryotic cell in the form of cystic lesions, fungal infections, virus infected cells, bacterial infections, autoimmune diseases, neoplasms and blood dyscrasias.
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Examples of Viral Inclusion Bodies
Acidophilic Intracytoplasmic Inclusion Bodies (eosinophilic)
Eg: Negri bodies in Rabies
Paschen bodies in Smallpox
Bollinger bodies in fowlpox
Acidophilic Intranuclear Inclusion Bodies (Eosinophilic)
Eg: Torres bodies in Yellow Fever
Cowdry type A in Herpes simplex virus
Cowdry type B in Polio and Adenovirus
Intranuclear Basophilic Inclusions
Eg: Cowdry type B in Adenovirus
Owl’s eye appearance - Cytomegalovirus
Both Intranuclear and Intracytoplasmic
Eg: Warthin-Finkeldey bodies in Measles
Viral Inclusion Bodies in Plants
Eg: Virus particles in Cucumber mosaic virus
Some of the Diseases that Involve Inclusion Bodies
Inclusion of body myositis which affects muscle cells.
Amyotrophic lateral sclerosis that affects motor neurons.
Dementia with lewy inclusions that affect cerebral neurons.
How to Prevent the Formation of Inclusion Bodies?
The cell inclusions are made of denatured aggregates of proteins (inactive), and several techniques have been developed to prevent the inclusion bodies formation. These help in the solubilisation and recovery of active proteins. The techniques are mentioned as follows:
Usage of weaker promoters to slow down the rate of protein expression.
Using low copy number plasmids.
Co-expression of chaperone.
Using specific E.coli strains.
Fusing the target protein with a soluble partner.
Lowering of the expression temperature.
FAQs on Inclusion Bodies: Structure, Classification & Significance
1. What are inclusion bodies according to the NCERT Class 11 syllabus?
According to the NCERT syllabus for Class 11, inclusion bodies are non-membrane-bound structures found freely in the cytoplasm of prokaryotic cells. Their primary role is to store reserve materials. These are not considered true organelles because they lack a lipid bilayer membrane and lie naked in the cytoplasm.
2. What is the main function of inclusion bodies in a bacterial cell?
The main function of inclusion bodies in bacteria is to act as cytoplasmic reservoirs. They store excess nutrients and energy sources that the cell does not immediately need. This includes storing carbon in the form of glycogen or poly-β-hydroxybutyrate (PHB), and inorganic phosphate as polyphosphate granules, ensuring the cell has resources during periods of limitation.
3. What are some common examples of inclusion bodies found in prokaryotes?
Prokaryotic cells contain several types of inclusion bodies, each with a specific storage function. Common examples include:
- Phosphate granules: Store inorganic phosphate (volutin).
- Cyanophycean granules: Store nitrogen reserves in the form of cyanophycin.
- Glycogen granules: Store glucose as a polymer for energy.
- Gas vacuoles: Provide buoyancy to aquatic photosynthetic bacteria.
- Carboxysomes: Contain the enzyme RuBisCO for carbon dioxide fixation.
- Sulphur granules: Store elemental sulphur, often as an energy reserve in sulphur bacteria.
4. How do inclusion bodies in prokaryotes differ from true organelles in eukaryotes?
The primary difference is that inclusion bodies are not enclosed by a lipid bilayer membrane, unlike eukaryotic organelles such as mitochondria, chloroplasts, or the nucleus. Inclusion bodies lie free in the cytoplasm. Furthermore, their function is mainly for passive storage, whereas eukaryotic organelles are complex, membrane-bound structures that perform active and specific metabolic processes like cellular respiration or photosynthesis.
5. What are gas vacuoles and why are they important for some bacteria?
Gas vacuoles are aggregates of hollow, protein-walled cylindrical structures called gas vesicles; they are not true vacuoles bound by a single membrane. Found in aquatic prokaryotes like cyanobacteria and green and purple photosynthetic bacteria, their primary importance is to provide buoyancy. By regulating their gas content, these bacteria can move up or down in the water column to position themselves at the optimal depth for light absorption and nutrient availability.
6. Why are carboxysomes considered a unique type of inclusion body?
Carboxysomes are unique because they have a more specialised and active metabolic role compared to simple storage granules. They are polyhedral bodies that contain a dense crystalline array of the enzyme Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). This compartmentalisation enhances the efficiency of carbon dioxide fixation in photosynthetic bacteria by concentrating CO₂ and minimising its wasteful reaction with oxygen (photorespiration).
7. What is the metabolic significance of storing phosphate and glycogen in bacterial inclusion bodies?
Storing these materials has significant metabolic advantages for survival. Glycogen granules serve as a readily available source of carbon and energy, allowing bacteria to survive periods of starvation. Phosphate granules (also known as volutin or metachromatic granules) act as a reserve of inorganic phosphate, which is crucial for synthesising ATP (the cell's energy currency) and nucleic acids (DNA and RNA), ensuring essential cellular processes can continue even when external phosphate is scarce.
8. Are the inclusion bodies found in bacteria a sign of disease?
No, in the context of bacteriology, inclusion bodies are normal physiological structures and are not a sign of disease. They are simply storage depots that the cell creates under specific nutrient conditions. This contrasts with the term's usage in virology or pathology, where 'inclusion bodies' can refer to abnormal accumulations of viral proteins or damaged cellular material within an infected or diseased eukaryotic cell.
