Bacterial Capsule and Virulence 3/6

Overview

This lecture explains the structure and importance of the bacterial capsule, its relationship to biofilms, and its role in microbial virulence.

The capsule is a type of glycocalyx, a sticky sugar coat produced by microorganisms.
Glycocalyx substances are gelatinous, polymeric mixtures of carbohydrates and/or proteins.
A loosely organized glycocalyx is called a "slime layer"; a tightly organized one is a "capsule."
Capsules are neatly organized layers that coat the surface of a bacterium.

Capsules increase a microorganism's virulence, making it more capable of causing disease.
The main reason for increased virulence is that capsules help bacteria evade phagocytosis (destruction by immune cells).
Immune cells like macrophages and neutrophils kill pathogens via phagocytosis, which capsules help bacteria resist.

Bacillus anthracis (anthrax) is only virulent if it has a capsule; without it, it cannot cause disease.
Streptococcus pneumoniae requires a capsule to cause pneumonia, sepsis, or meningitis; without it, it is not pathogenic.
Klebsiella pneumoniae is another bacterium whose pathogenicity depends on the presence of a capsule.

A capsule acts similarly to a single-organism biofilm, providing protection to the bacterium.
Biofilms are communities of microorganisms encased in a similar sticky coating, responsible for about 70% of human infections.

Glycocalyx — A sticky, gelatinous outer coating of a cell, composed of polysaccharides and/or proteins.
Capsule — A well-organized, tightly attached form of glycocalyx that surrounds a bacterium.
Slime layer — A loosely attached, unorganized glycocalyx.
Virulence — The degree to which a microorganism can cause disease.
Phagocytosis — The process by which immune cells engulf and destroy microorganisms.
Biofilm — A community of microorganisms encased in a self-produced sticky layer.