Lecture 6 - Virulence factors

Question 1

How are biofilm coated surfaces colonised.

Answer 1

In a compromised host there might be bare areas with exposed adhesion proteins
In a healthy host there will be no bare areas so the adhesion proteins are hidden. Therefore the pathogen needs to penetrate the biofilm to reach the receptor. The pathogen uses extracellular enzymes which dissolve the matrix to allow passage to the receptor (this process is not QS controlled)
The incoming pathogen will choose the path of least resistance where the biofilm is thinnest.

Once the bacteria have made it through the biofilm they dock to the surface using adhesins.

Pathogenic bacteria don’t immediately attack. Instead they sue quorum sensing to count themselves and others (others with a generic autoinducer AI-2) and when quorum is reached further virulence factors are expressed or upregulated.

Question 2

Define viurlence and pathogenisity.

Answer 2

Virulence refers to the degree of damage caused by a microbe to its host.
The pathogenicity is the ability to cause disease and is determined by its virulence factors.

Question 3

What are invasins

Answer 3

Invasins (QS controlled)
Some bacteria are able to penetrate into or between cells. This is accomplished by extracellular proteins (enzymes) called invasins. They act to break down host cells in the immediate vicinity of bacterial growth.
E.g.
* Hyaluronidase - commonly produced by Gram +ve bacteria such as streptococci, staphylococci and clostridia
It attacks the interstitial cement of connective tissue by depolymerising hyaluronic acid.
* Neuraminidase - produced by intestinal pathogens such as vibrio cholerae and shigella dysenteriae
Degrades neuraminic acid, an intercellular cement of the epithelial cells of the intestinal mucosa
* Streptokinase and staphylokinase - produced by streptococci and staphylococci respectively.
Converts inactive plasminogen to plasmin which digests fibrin and prevents clotting of the blood allowing more rapid diffusion of the infectious bacteria. As it can dissolve clots it is a cost effective and inexpensive thrombolysis medication.
* Collagenase - produced by clostridium histolyticum and clostridium perfringens
Breaks down collagen, the framework of muscles which facilitates gas gangrene in tissue.
* Coagulase - produced by staphylococcus aureus
Makes more collagen so the pathogen stays localised. (their adhesins bind to collagen).

Question 4

What are the two types of bacterial toxin?

Answer 4

There are two types of bacterial toxins endotoxin and exotoxin
1. Endotoxin (LPS): bound to cell
Released when bacteria lyse. Its action is indirect activating many host systems that cause damage.
2. Exotoxins
Actively secreted proteins that act on specific targets (e.g. protein synthesis)

Question 5

What are endotoxins.

Answer 5

Endotoxins
Endotoxins are the lipopolysaccharides present in the outer membrane of Gram - ve cell walls (not QS controlled).
Lipopolysaccharides consist of three covalently linked subunits.
* The polysaccharide component is made up of the O-antigen and core. It is water soluble and immunogenic.
* Lipid A is the toxic component normally embedded in the membrane (where it can’t cause harm0 however when the bacteria dies lipid A is released.
The LD50 is around 200-400 ug per animal which is weaker than exotoxins
Endotoxins cause host cells to release proteins (endogenous pyrogens) which affects the hypothalamus (temp control). This activates many host systems that cause damage leading to:
* Fever, shock, diarrhoea, vomiting, blood coagulation
* Weakness and inflammation
* However it is rarely fatal
Endotoxins cause problems in the pharmaceutical industry as drugs contaminated with endotoxins can cause complications. To test for endotoxins an in vitro limulus amoebocyte lysate (LAL) assay is used where the product is mixed with cells and the level of coagulation is measured.

Question 6

What are exotoxins?

Answer 6

Exotoxins

1. Cytolytic toxins - attack cell constituents causing lysis
   Cytolytic toxins cause disruption either by inserting into the cytoplasmic membrane and forming a pore or enzymatic attack of phospholipids. They are most easily detected via the breakdown of red blood cells (haemolysis) but they can attack other cells (leukocidin)
2. A-B toxins - two subunits, A and B. B binds to a cell surface receptor, A is then transported into the cytoplasm where it interferes with cell functions
   Entry into the cell is allowed via the specific binding of B-subunit to glycans. The A-subunit enters the cytosol and the B-subunit leaves the receptor. The A-subunit interferes with normal cell function (most inhibit protein synthesis)

Superantigens - stimulate large numbers of immune cells: damage by extensive inflammatory reaction