Morphological Characteristics and Classification
Surface mucous structures of a bacterial cell are generally divided based on their thickness and microscopic visibility.
Macrocapsules are prominent mucous layers firmly attached to the cell wall with distinct borders. The key dimensional criterion for a macrocapsule is a thickness exceeding 0.2 µm. These dimensions allow the structure to be easily detected using standard light microscopy with special staining techniques.
In contrast, the microcapsule is a similar surface mucous formation, but its layer thickness is less than 0.2 µm. Due to these extremely small dimensions, a microcapsule cannot be resolved with a standard light microscope; electron microscopy is required for its visualization.
Chemical Composition
From a biochemical perspective, the primary building blocks of bacterial capsules are carbohydrates, specifically exopolysaccharides. They form a dense hydrophilic scaffold around the cell.
However, microbiology features important exceptions. The most notable clinical example is the anthrax causative agent, Bacillus anthracis. Unlike the vast majority of other microorganisms, the capsule of this bacillus has a polypeptide nature: it is composed of polymeric molecules of D-glutamic acid. This confers specific properties and plays a vital role in disease pathogenesis.
Slime Layer 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 layer consists of mucoid exopolysaccharides that, unlike the capsule, lack distinct outer boundaries and easily dissolve in an aqueous environment.
- Glycocalyx is a specific term for exopolysaccharides whose main task is to ensure adhesion, meaning the attachment of the microbial cell to various surfaces and substrates.
These substances can be formed in two ways. The first is direct synthesis of exopolysaccharides by the microbial cell itself. The second is the production of extracellular enzymes by bacteria that cleave disaccharides from the surrounding environment, generating polymers such as levans and dextrans.
A striking clinical example of intensive slime production is found in mucoid strains of Pseudomonas aeruginosa. These microorganisms produce copious amounts of slime and are very frequently isolated from the sputum of patients suffering from cystic fibrosis.
Protective Functions of Surface Structures
Both the capsule and slime layers play an essential role in bacterial survival, providing complex protection through their unique physicochemical properties.
- Protection against desiccation. Due to its pronounced hydrophilicity, the capsule can retain significant volumes of water inside, saving the bacterial cell from moisture loss in adverse conditions.
- Mechanical protection. The mucous layer acts as a shock absorber, protecting fragile cell structures from direct physical damage.
- Biological protection. This is arguably the most significant function from a medical standpoint. The capsule serves as a powerful barrier against the host immune system. Its key task is to prevent phagocytosis, stopping immune cells from engulfing and destroying the bacterium. Additionally, this layer reliably protects the cell from penetration by specific viruses, known as bacteriophages.
Laboratory Diagnostics and Antigenic Properties
Full capsule formation strongly depends on the bacterial habitat. As a rule, capsules are robustly formed directly within the host organism (easily observed when examining smears of pathological material). Conversely, 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-Gins method is traditionally used. This method relies on negative staining. During staining, India ink is applied to create a dark, impermeable background. The capsule itself does not take up the dye and remains transparent, allowing it to be observed under the microscope as a distinct light halo surrounding the bacterial body.
Furthermore, the capsule exhibits pronounced antigenic properties. If specific antibodies directed against capsular antigens are added to microorganisms, a phenomenon known as the capsular swelling reaction (Quellung reaction) occurs. This phenomenon is actively used in microbiological practice for the precise identification of various bacterial species.