Bacterial capsule

Morphological characteristics and classification

Surface mucous structures of the bacterial cell are conventionally divided according to their thickness and ability to be visualized under a microscope.

A macrocapsule is a distinct mucous layer that is firmly attached to the cell wall and has clear outlines. The main size criterion for identifying it is a thickness exceeding 0.2 µm. Such dimensions make it easy to detect this structure using standard light microscopy with special staining techniques.

In contrast, there is the microcapsule. This is a similar mucous formation on the cell surface, but its layer thickness is less than 0.2 µm. Due to such small dimensions, the microcapsule cannot be resolved under a conventional light microscope; electron microscopy methods are required for its detection.

Chemical composition

From a biochemical standpoint, carbohydrates serve as the main building material of bacterial capsules, specifically exopolysaccharides. They form a dense hydrophilic framework around the cell.

However, there are important exceptions in microbiology. The most famous clinical example is the anthrax causative agent, Bacillus anthracis. Unlike the vast majority of other microorganisms, the capsule of this bacillus is polypeptide in nature: it is built from polymeric molecules of D-glutamic acid. This gives it specific properties and plays an important role in the pathogenesis of the disease.

Slime and glycocalyx: differences from the true capsule

In addition to the tightly bound capsule, bacteria can form less structured surface layers, such as slime and glycocalyx.

• Slime refers to mucoid exopolysaccharides which, unlike the capsule, lack distinct outer boundaries and easily dissolve in an aqueous environment.
• The glycocalyx is a specific term designating those exopolysaccharides whose main task is to ensure adhesion, i.e., the attachment of the microbial cell to various surfaces and substrates.

The formation of these substances can occur in two ways. The first is the direct synthesis of exopolysaccharides by the microbial cell itself. The second is the production by bacteria of extracellular enzymes that cleave disaccharides from the environment, resulting in the formation of polymers such as levans and dextrans.

A striking clinical example of intensive slime production is mucoid strains of Pseudomonas aeruginosa. These microorganisms produce large amounts of slime and are extremely frequently isolated from the sputum of patients suffering from cystic fibrosis.

Protective functions of surface structures

Both the capsule and mucous layers play a crucial role in bacterial survival, providing comprehensive protection thanks to their specific physicochemical characteristics.

1. Protection against desiccation. Due to pronounced hydrophilicity, the capsule is able to retain significant volumes of water within itself. This saves the bacterial cell from moisture loss under unfavorable conditions.
2. Mechanical protection. The mucous layer acts as a shock absorber, protecting fragile cell structures from direct physical damage.
3. Biological protection. This is perhaps the most significant function from a medical perspective. The capsule becomes a powerful barrier against the host immune system factors. Its key task is to prevent phagocytosis, stopping immune cells from capturing and destroying the bacterium. Furthermore, this layer reliably protects the cell from the penetration of specific viruses—bacteriophages.

Laboratory diagnostics and antigenic properties

Full capsule formation heavily depends on the bacterial habitat conditions. As a rule, it forms exceptionally well directly inside the host organism (which is easily noticed when examining smears from pathological material). When microorganisms are grown in pure cultures on artificial nutrient media, capsule formation occurs much less frequently.

To visualize the capsule in the laboratory, the Burri-Hins method is traditionally used. This method is based on the principle of negative contrast. During staining, India ink is applied, creating a dark impermeable background. The capsule itself does not take up the stain and remains transparent, allowing it to be observed under the microscope as a distinct light halo surrounding the bacterial body.

Additionally, the capsule exhibits pronounced antigenic properties. If specific antibodies directed against capsular antigens are added to microorganisms, the so-called capsular swelling reaction occurs. This phenomenon is actively used in microbiological practice for the precise identification of various bacterial species.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of dextran and levan formation under the action of extracellular enzymes
• Pathogenesis of mucoid strains of Pseudomonas aeruginosa in cystic fibrosis
• Immunological mechanisms of the capsular swelling reaction
• Molecular bases of the antiphagocytic activity of capsular polysaccharides

Other topics in the section "Morphology and Classification of Microbes"

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