Question

What is one virulence factor that is effective in enabling bacteria to cause disease? Explain what this virulence factor is and how it helps the bacteria and subsequently harms the host.

Answer 1

Virulence factors help bacteria to

(1) invade the host,

(2) cause disease, and

(3) evade host defenses

There are many virulence factors present in bacteria but the capsule plays a pivotal role in bacterial pathogenicity.

Polysaccharide capsules, in prokaryotes characteristically composed of repeating oligosaccharides, are found on the surface of many bacteria. These capsules are typically composed of only one polysaccharide and lie outside the outer membrane of gram-negative cells and the peptidoglycan layer of gram-positive cells. In general, individual bacteria do not exhibit variation of these antigens as has been described for the variant glycoproteins of trypanosomes . Comprising 99 % water, these highly hydrated, polyanionic polysaccharide capsules serve many functions. These include determining access of molecules and ions to the bacterial cell envelope and the cytoplasmic membrane, the promotion of adherence to the surfaces of inanimate objects or living cells and the formation of biofilms and microcolonies .

Capsulate bacteria are responsible for causing some of the most serious invasive infections to which man is susceptible, including septicaemia, meningitis, pneumonia, osteomyelitis, septic arthritis and pyelonephritis.

Mechanisms by Which Capsular Polysaccharides Enhance Virulence

Most capsular polysaccharides are hydrophilic and confer a negative charge on the bacterial cell, characteristics which are intrinsically anti phagocytic in their effect, that the hydrophilic properties of polysaccharide capsules act by reducing the surface tension at the interface between the phagocytic cell and the bacterium and that this impairs the facility with which phagocytic ingestion occurs.

The capsule is considered a virulence factor because it enhances the ability of bacteria to cause disease (e.g. prevents phagocytosis). The capsule can protect cells from engulfment by eukaryotic cells, such as macrophages. A capsule-specific antibody may be required for phagocytosis to occur. Capsules also contain water which protects the bacteria against desiccation. They also exclude bacterial viruses and most hydrophobic toxic materials such as detergents. Immunity to one capsule type does not result in immunity to the other types. Capsules also help cells adhere to surfaces. As a group where the capsule is present they are known as polysaccharide encapsulated bacteria or encapsulated bacteria.