Slime layer and capsule, together summarised as glycocalyx, are extracellular materials secreted by some bacterial cells. Slimer layer and bacterial capsule are sometimes used interchangeably, but they are totally different structures. This article focuses on describing and highlighting the differences between slime layer and capsule.

Table of Contents

• Slime Layer
• Function
• Capsule
• Function
• Slime Layer vs Capsule
• Frequently Asked Questions

Slime Layer

A slime layer is an unorganised loosely associated extracellular layer that surrounds the bacterial cell wall. It is made up of glycoproteins, glycolipids and exopolysaccharides. Slime layers are amorphous in nature and are of varied thickness because they are produced depending on the cell type and environment. The layers are found hanging like strands extracellularly forming a net-like anatomy between cells that are 1-4 μm apart. Because they are loosely associated with the bacterial cell wall, the slime layer can be easily washed off.

Function

• It protects the bacterial cell from physical damage such as desiccation and antibiotics.
• It helps the bacteria in adhering to smooth surfaces.
• A slime layer is mainly composed of polysaccharides and hence is overproduced in unfavourable times as extra food storage for survival.
• It is also produced in soil dwelling prokaryotes to prevent them from unnecessary drying during annual temperature and humidity shifts.
• It sometimes helps the bacteria to survive sterilisation by chemicals such as iodine and chlorine, but it cannot escape autoclaving.
• Some research studies show that slime layers also play a role in immunity by absorbing antibodies.

Capsule

A bacterial capsule is an organised and tightly associated extracellular layer present around the bacterial cell wall. It is made up of simple sugars or polysaccharides. Unlike the slime layer, it is tightly packed and hence cannot be easily washed off. It can be found in both gram positive and gram negative bacteria. When the amorphous secretion of a capsule diffuses into the surrounding medium and becomes loose, it is called a slime layer.

The bacterial capsule is mainly composed of polysaccharides but they might have some exceptions, example being Bacillus anthracis which also has poly-D-glutamic acid. Capsules can be viewed under a microscope by staining the bacterial cell with India ink. The tight structural network of the capsule prevents it from being stained and thus it appears like a bright halo against a dark background.

Function

• The capsules are water loving (hydrophilic) and hence prevent the bacterial cell from water loss or desiccation.
• It also protects the bacterial cell wall from engulfment by the white blood cells (phagocytosis).
• The presence of a capsule in bacteria determines its virulence factor. For example, Pseudomonas aeruginosa, which causes cystic fibrosis, produces a thick capsular layer of alginic acid upon invading the lung cells that makes it difficult for our immune system to eradicate it.
• It also helps the bacteria to adhere to various surfaces.

Slime Layer vs Capsule

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Also see:

• Do All Bacteria Have Plasmids?
• Human Bacterial Diseases
• Difference between Aerobic and Anaerobic Bacteria