Capsules

You must have noticed that it splits up and forms seeds whenever a fruit is mature. But is this phenomenon as simple as it sounds? There is more to this process. This fruit which later splits up to form seeds, is called a capsule. In microbiology, the case of a capsule is different. In this area, capsules are a big structure of bacteria that can cause many diseases. Let’s learn more about them.

Capsules meaning

These capsules are bacterial and are made up of polysaccharides located on the outer area of the cell membrane. Capsules can also be called ‘Glycocalyx.’ This capsule can also be seen under a light microscope. You will notice that around the cell wall, an envelope is created. A Capsule creates this envelope-like structure. A capsule is made up of many different types of sugars or polypeptides. The nature of these Capsules is sticky.  

Functions of Capsules

Capsules have several functions whenever they are related to bacteria. Some functions of the capsule are given below: 

• These capsules can prevent the attachment of bacteriophages.
• Capsules are hygroscopic, which means they contain water molecules. As an advantage to bacterial cells, capsules protect them from desiccation.
• They have sticky properties and can survive in the environment because of this property. After they stick, they can grow into bigger systems in many places like human teeth, fast water streams, plants roots, fruits, vegetables, etc. 
• Capsules are used as a source of energy by S. Mutans. The sugars inside the capsule are broken by the S. Mutans when there is a low energy level. 
• The Capsules maintain the viscosity of the cell. They also protect the cells from getting desiccated. They regulate the movements of the nutrients from different places of the bacterial cells. 
• With the help of a capsule, the cell is protected by phagocytosis. 
• Capsules help to protect the cells from extra outside damage and chemical danger.
• They help prevent the attack of bacteriophages from the cell’s surface.
• The Capsules protect the anaerobic cells from oxygen toxicity.
• Capsules prevent the cells from dying.  

Capsules composition

Capsules are made up of polysaccharides and other kinds of sugars. In some cases, they are composed of other materials such as poly-D-glutamic acid (capsules of Bacillus anthracis), etc. 

Staining capsules is difficult because their layers are tightly packed. Capsules need dark inks so that they can be easily studied under the microscope.

Types of Capsules

  Capsules are of two types.

• Macro capsules- the thickness of these capsules are more than 0.2um, and they can be seen under a light microscope.
•  Microcapsules- the thickness of these capsules is 0.2um. They are visible only under an electron microscope. 

Capsules examples (gram-positive bacteria)

• Streptococcus agalactiae
• Staphylococcus epidermidis
• Bacillus anthracis
• Streptococcus pyogenes
• Streptococcus pneumoniae
• Bacillus megaterium 

 Capsules examples (gram-negative bacteria) 

• Escherichia coli (in some strains) 

• Salmonella
• Klebsiella pneumoniae
• Pseudomonas aeruginosa
• Haemophilus influenzae
• Neisseria meningitidis

 Bacterial capsule and slime layer  difference

• Definition

A sugar or layer of glycocalyx, which is constructed of polysaccharide   molecules, is called a capsule 

The Slime layer is a layer of glycocalyx that is loosely made up of glycoprotein molecules.   

• Binding 

Capsules are tightly held to the cell wall of the cell.

The Slime layer is loosely held to the cell wall of the cell. 

•  Organisation

The layers of capsules are well organised.

The layers of slime are not well organised.

•  Washed off ability 

The layers of the capsule are difficult to wash off.

The layers of slime are easy to wash off. 

•  Roles 

Capsules act as a virulence factor.

The main role of the slime layer is the protection of cells from nutrient loss and dehydration.

• Thickness 

The layer of the capsule is thicker than the slime layer.

The layer of slime is a thin glycocalyx layer.

• Composition 

The capsule is made up of polysaccharides.

The slime layer is made up of exopolysaccharides, glycoproteins, and glycolipids.

Conclusion 

Capsules have different terminology in different fields of biology. A capsule has a large structure which is common in many bacteria. These layers are difficult to wash off, making of sugars. They can either be made up of simple sugars or complex sugars. It has many functions when it comes to its uses for bacteria. This layer is organised well. Capsules are very small to be seen with naked eyes; they are stained first and then seen under microscopes. They can not be seen if they are stained with simple inks. They have to be stained with dark inks.
In some cases, vaccinations are also done with some materials with capsules. They are not antigenic, especially for kids. Many vaccines that have capsules are made up of polysaccharides with protein carriers. Some examples of carriers of proteins are diphtheria toxoid, tetanus toxoid, etc. They stimulate the immune system. They can also be seen with a specific serum that is anticapsular. Then if you notice these capsules under the microscope, you can notice that the capsules are swollen. This is because the refractivity of the capsules has increased.