Virulence factors, extracellular products, and toxins

Bacterial virulence factors enable them to replicate and disseminate within a host in part by subverting or eluding host defences. Bacteria are armed with different types of virulence factors. We will take a look at them below.

Adherence Factors

They help bacteria to attach or adhere to host tissues. In most cases this is the first step in the pathogenesis of an infection or colonization. They can be the following

1. Pili: In E.coli., Neisseria gonorrhoeae pili mediate adherence to urinary tract epithelium

2. Glycocalyx: In S.epidermidis and S.viridans it helps to attach to heart valves.

3. Curli: They are amyloid protein fibre like extensions projecting from the cell wall of some bacteria. In E.coli and Salmonella they mediate binding to endothelium and extracellular proteins like fibronectin. Curli bind to Factor XII facilitating thrombus formation in DIC associated with sepsis.

Enzymes helping in invasion

These bacterial enzymes help in breaking down tissue barriers and host defenses and thus spreading the infection. The important ones are as follows

1. Collagenase and Hyaluronidase: important in the pathogenesis of cellulitis in Streptococcus pyogenes necrotizing fasciitis giving them the nickname of “flesh eating bacteria”.

2. Coagulase: Walls off the infected area with fibrin to protect from phagocytes. Seen commonly in Staphylococcus aureus.

3. IgA Protease: Breaks down mucosal immunoglobulin Ig A allowing bacteria to adhere to mucosa. Seen in Neisseria gonorrhoeae, Haemophilus influenzae and Streptococcus pneumoniae.

4. Leukocidins: These are enzymes that destroy neutrophils and macrophages.

Capsule

The capsule prevents phagocytes from adhering to the bacteria and protects them from phagocytosis and opsonization. e.g.Streptococcus pneumoniae and Neisseria meningitidis.

Biofilms

A biofilm is defined as an assemblage of microbial cells that is irreversibly associated (not removed by gentle rinsing) with a surface and enclosed in a matrix of primarily polysaccharide material. They protect bacteria from antibiotics and host defenses like neutrophils and antibodies. They are often seen in device associated infections like indwelling urinary catheters, central lines and intraperitoneal catheters, native and prosthetic valve infections and dental plaques. Pseudomonas aeruginosa biofilms are seen in patients with Cystic Fibrosis

Cell Wall Proteins

M protein of group A Streptococcus is antiphagocytic and protein A of Staphylococcus aureus binds to IgG and prevents activation of complement.

Intracellular survival

Some bacteria evade the human immune system by hiding and surviving within the host cell.e.g. Mycobacteria, Listeria, Brucella and Legionella. Mycobacteria and Legionella inhibit fusion of the phagosome with lysosome thus escaping from degradation. They also inhibit acidification of the phagosome which is essential for digestion of internalized bacteria in the phagosome. Listeria escapes from the phagosome into the cytoplasm hence avoiding degradation by lysosomal enzymes.

Endotoxins

Endotoxins are produced by Gram negative bacteria. They are Lipopolysaccharides (LPS) which are normally present in the cell wall of Gram negative bacteria. The toxic portion of the molecule is called Lipid A.Endotoxin is encoded by genes on the bacterial chromosome. Endotoxins cause fever, septic shock and DIC leading to SIRS ( systemic inflammatory response syndrome).

Effects of Endotoxin and Mediators Involved

Mechanism of action of Endotoxin

Endotoxin binds to LPS- binding protein in the serum, which then binds to CD 14 on the surface of the macrophage. CD 14 interacts with Toll Like Receptor or TLR (a transmembrane protein) which stimulates intracellular signaling cascade leading to activation of genes coding for various cytokines like Interleukins and TNF.

Exotoxins

Exotoxins are polypeptides secreted by both gram positive and gram negative bacteria. The genes coding for exotoxins are located on plasmids or lysogenic bacteriophages. When treated with formaldehyde, acid or heat, they are converted to toxoids. This property is used in the development of toxoid vaccines. Exotoxins are secreted by bacteria by specialized secretion systems. Type III secretion system is most important, called an injectosome. It is important for the virulence of Pseudomonas aeruginosa, Shigella spp., E.coli, Salmonella spp. and Yersinis pestis.

Medically important exotoxins and their characteristics