Difference Between Capsule and Slime Layer

Main Difference – Capsule vs Slime layer

Capsule and slime layer are two structures that are found in the outside cell wall of many bacteria. Capsule allows bacteria to invade the immune system of the host. Hence, it is common in parasitic bacterial forms. Both capsule and slim layer are composed of a sugar shell called glycocalyx. Glycocalyx layer is considered as an additional layer to the cell wall. The main difference between capsule and slime layer is that capsule is a thick glycocalyx layer that is tightly bound to the cell, defining boundaries of the cell whereas slime layer is a thin glycocalyx layer that is loosely bound to the cell.

This article explores,

1. What is Capsule 
      – Structure, Characteristics, Role
2. What is Slime Layer
      – Structure, Characteristics, Role
3. What is the difference between Capsule and Slime Layer

What is Capsule

The capsule is a polysaccharide layer that is a deemed part of the cell envelope of most prokaryotes. It occurs in both gram positive and gram negative bacteria. The capsule is a well-organized layer, which cannot be easily washed out. Therefore, it can cause diseases in other organisms. Streptococcus pneumoniae allows the bacterium to cause pneumonia due to the presence of a capsule. The capsule can be visualized under the light microscope by combining the cells with India ink. It appears as a halo surrounding the cell by excluding the ink. The virulence factor of bacteria is increased by the capsule. The capsule also protects the bacterium from dehydration and detergents. A photomicrograph of Streptococcus pneumoniae is shown in figure 1. The bacterial capsule exhibits a halo appearance.

Figure 1: Photomicrograph of Streptococcus pneumoniae

What is Slime Layer

The slime layer is composed of loosely-packed, diffused, unorganized extracellular material that surrounds the bacterial cell. It traps nutrients while aiding in the cell motility. It binds cells together and adheres to smooth surfaces. Slime layer is composed of exopolysaccharides, glycoproteins, and glycolipids. It can be easily removable by centrifugation. In the presence of sucrose, Streptococcus mutans produces a slime layer, allowing other bacteria to aggregate to the tooth surfaces. This results in dental plaques. Capsule and slime layer in bacteria are shown in figure 2.

Figure 2: Capsule and slime layer
1 – Capsule, 2 – Slime layer

Difference Between Capsule and Slime Layer

Definition

Capsule: A glycocalyx layer, consisting of firmly associated polysaccharide molecules with the cell wall is called the capsule. 

Slime Layer: A glycocalyx layer that consists of loosely associated glycoprotein molecules is called the slime layer.

Composition 

Capsule: Capsule is composed of polysaccharides.

Slime Layer: Slime layer is composed of exopolysaccharides, glycoproteins, and glycolipids.

Thickness

Capsule: Capsule is thicker than the slime layer.

Slime Layer: Slime layer is a thin glycocalyx layer.

Binding

Capsule: Capsule is tightly bound to the cell wall.

Slime Layer: Slime layer is loosely bound to the cell wall.

Organization

Capsule: Capsule is a well-organized layer. Hence, it is difficult to be washed off.

Slime Layer: Slime layer is an unorganized layer. Hence, it can be easily washed off.

Role

Capsule: Capsule acts as a virulence factor that helps to evade phagocytosis.

Slime Layer: Slime layer mainly aids in the adherence. It also protects the cell from dehydration and nutrient loss.

Conclusion

Slime layer is a loose, gelatinous layer deposited around the bacterial cell wall. It is mainly composed of polysaccharides. Slime layer become thicker due to the deposition of nitrogen compounds along with the slime layer, forming a capsule. A capsule is common in parasitic bacterial species. The main difference between capsule and slime layer is the organization of each glycocalyx layer.

Reference:
1. Rogers, Kara, and Robert J. Kadner. “Bacteria.” Encyclopædia Britannica. Encyclopædia Britannica, inc., 24 Apr. 2017. Web. 20 May 2017. <https://www.britannica.com/science/bacteria/Capsules-and-slime-layers>.
2. Microscope, Through The. “-32 Bacterial cells are often covered in glycocalyx.” Through the Microscope Main News RSS. N.p., n.d. Web. 20 May 2017. <http://www.microbiologytext.com/5th_ed/book/displayarticle/aid/67>.

Image Courtesy:
1.”Pneumococcus CDC PHIL 2113″ from the Centers for Disease Control and Prevention’s Public Health Image Library (PHIL) (Public Domain) via Commons Wikimedia
2. “Bacterial mucoid diagram” By Y tambe – Y tambe’s file (CC BY-SA 3.0) via Commons Wikimedia