Difference Between Capsule and Glycocalyx

The key difference between capsule and glycocalyx is that capsule is an organized, well defined, condensed extracellular layer that is tightly bound to the cell envelope of bacteria, while glycocalyx is an additional layer composed of polysaccharides and/or polypeptides outside the cell wall of bacteria.

Some bacteria have an additional layer called glycocalyx outside the cell wall. It is made of extracellular materials. It protects bacteria from external conditions and helps in adhering to surfaces. Glycocalyx exists in two forms: slime layer or capsule. Slime layer is the extracellular layer that is loosely associated with the bacterial cell wall. It is a less discrete layer that can be easily washed off. Capsule is attached tightly to the cell wall, and it is a thick discrete layer. Capsule cannot be easily removed from the bacteria. Both slime layer and capsule help bacteria from desiccation and antimicrobial agents. Most of the encapsulated bacteria are pathogenic, and they avoid phagocytosis due to their glycocalyx.

CONTENTS

1. Overview and Key Difference
2. What is Capsule 
3. What is Glycocalyx
4. Similarities Between Capsule and Glycocalyx
5. Side by Side Comparison – Capsule vs Glycocalyx in Tabular Form
6. Summary

What is Capsule?

Capsule is one of the external structures possessed by some bacteria. They are made from polymers of polysaccharides. Capsule is an organized structure that surrounds the cell envelope of bacteria, and it is tightly bound to the cell envelope. Hence, it is very hard to wash off. Capsule is thick and aids bacteria to avoid phagocytosis. Moreover, capsules are hydrophilic in nature. Hence, it prevents bacteria from desiccation.

The production of the capsule is genetically controlled and subjected to environmental modification. The density, thickness and adhesiveness of the capsules vary among different bacterial strains. Furthermore, the chemical composition of the capsule varies among bacterial species. They may consist of glucose polymers, complex polysaccharides, aminosugars, sugar acids, and polypeptides alone or in combination.

Capsule is considered as a virulence factor of bacteria due to its ability to escape from the host defence mechanisms causing diseases. Straphylococcus aureus is a bacterial species that resist neutrophil phagocytosis due to its capsule. The capsule of the Streptococcus pneumoniae is the key factor to cause pneumonia. It is observed that the loss of capsule reduces the virulence of bacteria.

Capsules have several functions. They often mediate adherence of cells to surfaces. Capsules also protect bacterial cells from engulfment by predatory protozoa or white blood cells or from attack by antimicrobial agents. Sometimes capsules become reservoirs of carbohydrates when bacteria are fed with sugars. Another important characteristic of capsules is the ability to block some steps of the phagocytosis process and thereby prevent bacterial cells from being engulfed or destroyed by phagocytes.

Capsules can be visualized by negative staining techniques using India ink under the microscope. Capsule will appear as clear halos surrounding the bacterial cells. Some examples of encapsulate bacteria are Bacillus antracis, Klebsiella pneumonia, Streptococcus pneumonia and Clostridium perfringens.

What is Glycocalyx?

Glycocalyx is an important structure found in some bacteria. Glycocalyx avoids bacterial cells from phagocytosis and helps the formation of biofilms. Glycocalyx exists in two forms as capsule and slime layer. The capsule is a highly organized, tightly bound thick glycocalyx that helps bacteria to escape phagocytosis. The Slime layer is an unorganized, loosely adhered thin glycocalyx that protects bacterial cells from drying. Moreover, the slime layer traps nutrients and aids in biofilm formation.

The slime layer is composed mostly of exopolysaccharides, glycoproteins, and glycolipids. It can be easily washed off due to its loose adhesion to the cell wall. Structurally, it is a loosely bound gelatinous extracellular layer.

What are the Similarities Between Capsule and Glycocalyx?

• A distinct, gelatinous glycocalyx is called a capsule.
• Therefore, capsule is one of the two forms of glycocalyx.

What is the Difference Between Capsule and Glycocalyx?

The glycocalyx is an additional layer outside the cell wall which exists in two forms: capsule and slime layer. Meanwhile, a capsule is the form of glycocalyx that is organized and attached tightly to the cell wall. So, this is the key difference between capsule and glycocalyx. Besides, the capsule is composed of polysaccharides while thin glycocalyx is composed of exopolysaccharides, glycoproteins, and glycolipids. Thus, this is another difference between capsule and glycocalyx.

Moreover, the capsule is firmly attached to the cell wall while the thin glycocalyx is loosely attached to the cell wall. Also, the capsule is not easily washed off while thin glycocalyx is washed off.

The below infographic tabulates the differences between capsule and glycocalyx in more detail.

Summary – Capsule vs Glycocalyx

The capsule is one of the two forms of the glycocalyx. Also, the capsule is attached tightly to the cell wall. Hence, it is difficult to wash off. Moreover, the presence of a capsule is a virulence factor of bacteria. The thin glycocalyx is known as the slime layer, and it is loosely attached to the cell wall. Thus, the thin glycocalyx can be easily washed off. Also, glycocalyx aids bacteria in avoiding phagocytosis and forming biofilms. Thus, this summarizes the difference between capsule and glycocalyx.

Reference:

1. “Capsules and Slime Layers.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., Available here.
2. “Bacterial Capsule.” Wikipedia, Wikimedia Foundation, 15 Feb. 2021, Available here.

Image Courtesy:

1. “OSC Microbio 03 03 Capsules” By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia
2. “Glycocalyx in bacteria” By Jsun15 – Own work (CC BY-SA 3.0) via Commons Wikimedia