Capsules and Slime Layers

Capsule Slime Layer

Many bacterial cells secrete a capsule or a slime layer as an extracellular material. A slime layer is loosely attached to the bacterium and can be washed away, while a capsule is firmly attached to the bacterium and has distinct boundaries.

This article will study slime layer and capsule, slime capsule definition and bacterial capsule and slime layer.

Slime Layer and Capsule

  • The capsule is a polysaccharide film that extends beyond the cell envelope and is thus considered part of the bacterial cell's outer envelope. It's a well-organized layer that's difficult to remove and can cause a variety of diseases. The capsule, which can be present in both gram-negative and gram-positive bacteria, is distinct from the second lipid membrane, the bacterial outer membrane, which only exists in gram-negative bacteria and contains lipopolysaccharides and lipoproteins. 

  • A slime layer is created when the amorphous viscid secretion (that makes up the capsule) diffuses into the surrounding medium and persists as a loose secretion. 

  • The glycocalyx is a term that refers to both the capsule and the slime layer.

Detailed Composition of Slime Capsule

Composition of Capsule

While most bacterial capsules are made of polysaccharides, some organisms, such as Bacillus anthracis, use other materials, such as poly-D-glutamic acid. Since most capsules are so tightly packed, most standard stains cannot penetrate the capsule, making them difficult to stain. A sample is stained with a dark dye, such as India ink, to make it easier to see encapsulated bacteria under a microscope. The capsule's arrangement prevents the stain from entering the cell. Bacterial capsules appear as a light halo around the cell against a dark background when viewed.

Functions of Capsule

The capsule is referred to as a virulence factor because it increases bacteria's capacity to cause disease (e.g. prevents phagocytosis). The capsule will protect cells from eukaryotic cells like macrophages engulfing them. [number six] Phagocytosis can involve the presence of a capsule-specific antibody. Water is also present in the capsules, which prevents the bacteria from drying out. Bacterial viruses and most hydrophobic toxic products, such as detergents, are also excluded.  Immunity to one form of capsule does not mean immunity to the others. Capsules also aid in the adhesion of cells to surfaces. Polysaccharide encapsulated bacteria are a group of bacteria that have a capsule.

Slime Layer of Bacteria

A slime layer in bacteria is an unorganized layer of extracellular material that covers bacteria cells and is easily removed (e.g. by centrifugation). Exopolysaccharides, glycoproteins, and glycolipids make up the majority of this. As a result, the slime layer is assumed to be a branch of the glycocalyx.

Although slime layers and capsules are most commonly found in bacteria, archaea may also have these structures. This structure and role knowledge are also transferable to these microorganisms.

Structure of Slime Layer

  • Slime layers are amorphous and vary in thickness, with different amounts formed depending on the cell type and environment. These layers appear as strands hanging extracellularly and forming net-like structures between cells separated by 1-4m. Researchers believe that after 9 days of development, a cell's formation of the slime layer will slow, possibly due to slower metabolic activity.

  • A bacterial capsule is similar to a slime layer, but it is more rigid. In comparison to their slime layer counterparts, capsules are more structured and difficult to extract. An S-layer is a highly structured but separate structure. S-layers are glycoprotein-based structures that incorporate themselves into the cell wall and provide rigidity and protection to the cells. The cell's rigidity is not aided by a slime layer because it is loose and flowing.

Functions of Slime Layer

  • The slime layer's job is to protect bacteria cells from environmental threats including antibiotics and desiccation. Bacteria can adhere to smooth surfaces like prosthetic implants and catheters, as well as other smooth surfaces like Petri dishes, due to the slime layer. The cells adhered to the culture vessel without any additional appendages, depending solely on extracellular material, according to the researchers.

  • A slime layer, though primarily composed of polysaccharides, can be overproduced to the point that the cell may rely on it as additional food storage during a famine. In addition, ground-dwelling prokaryotes can produce a slime layer to prevent excessive drying caused by annual temperature and humidity changes.

  • Bacterial colonies can be able to withstand chemical sterilization with chlorine, iodine, and other chemicals, leaving autoclaving or boiling water flushing as the only surefire method of decontamination.

Difference Between Capsule and Slime Layer


Slime Layer 

It is a glycocalyx layer consisting of tightly associated polysaccharide molecules with the cell wall.

It is a glycocalyx layer consisting of loosely associated glycoprotein molecules.

The capsule is composed of polysaccharides.

The Slime layer is composed of glycoprotein, glycolipids, and exopolysaccharide.

It is thicker than the slime layer.

It is a thin layer.

It is tightly bound to the cell wall

It is loosely bound to the cell wall.

Well organized layer

Unorganized layer

Acts as a virulence factor.

It helps the cell to prevent dehydration and nutrient loss.

Did You Know?

Since there are so many bacteria that are becoming immune to antimicrobial agents like antibiotics (which prevent cell growth or simply destroy the cell), new research is being published on new drugs that reduce virulence factors in certain bacteria. Anti-virulent drugs inhibit bacteria's pathogenic properties, enabling the host to fight them or antimicrobial agents to function. Staphylococcus aureus is a pathogenic bacteria that causes a variety of human infections and possesses a variety of virulence factors, including biofilm formation, quorum sensing, and exotoxins, to name a few. 

FAQs (Frequently Asked Questions)

1. What are the Functions of Bacteria Capsules and Slime Layers?

Ans: A slime layer is a non-rigid matrix that can be easily deformed and cannot keep India Ink out. Many cells and their outer barriers make up biofilms. Both capsules and slime layers have two primary functions: defense and adhesion.

2. What is the Composition of the Slime Layer?

Ans: A slime layer in bacteria is an unorganized layer of extracellular material that covers bacteria cells and is easily removed (e.g. by centrifugation). Exopolysaccharides, glycoproteins, and glycolipids make up the majority of this. As a result, the slime layer is assumed to be a branch of the glycocalyx.

3. Is Glycocalyx Present in All Bacteria?

Ans: The glycocalyx is a viscous outer layer of fibres that spreads from the bacterium that is secreted by all bacteria. A capsule is a large, tightly bound glycocalyx that adheres to the cell wall.