Exotoxins

Definition:

Protein toxins usually secreted from a living bacterium but also released upon bacterial lysis.

Exotoxins divided into three types on the basis of their structure and activities:

1. 
   
   A-B toxins.
   
   
   
2. 
   
   Membrane disrupting Toxins "Cytolytic enzymes".
   
   
   
3. 
   
   Superantigens.
   
   
   

(1) A-B toxins:

These toxins-interfere with cell metabolism.

Each is formed of two portions:

Portion A:

 It is the enzymatic portion of the toxin.

 It is translocated into host cells after binding of B portion.

Portion B:

 Binds to specific host cell.

Binding and Entry of an A-B Toxin

A-B toxins consist of two parts, an A (active) component and a B (binding) component. The B component of the exotoxin binds to a receptor on the surface of a susceptible host cell. The exotoxin now enters the host cell, in this case by endocytosis, and causes harm by inactivating a host cell target protein through ADP-ribosylation.

Examples:

• 
  
  Diphtheria exotoxin, produced by Corynebacterium diphtheriae.
  
  
  
• 
  
  Cholera exotoxin, produced by Vibrio cholerae.
  
  
  
• 
  
  Botulinum exotoxin, produced by Clostridium botulinum.
  
  
  

(2) Membrane disrupting Toxins "Cytolytic enzymes":

This class of exotoxin lyses host cells by disrupting the plasma membrane.

Two kinds are known:

One is a protein that forms pores in the membrane causing leakage of cytoplasm and water entry.

The other acts as phospholipase.

These toxins are general cytotoxins because they can affect any host cell.

However, they are also referred to as "hemolysins" due to their effect on erythrocytes, which are a convenient model of their actions.

Examples:

• 
  
  Listeriolysin: it is a pore-forming cytotoxin that is produced by Listeria monocytogenes.
  
  
  
• 
  
  Lecithinase: it is a phospholipase that is produced by Clostridium perfringens.
  
  
  

(3) Superantigens:

Bind directly to the outside of MHC-II molecules and are recognized by many CD4 T-lymphocytes.

This non-specific activation of very large numbers of these lymphocytes results in a release of large amount of interleukin-2 and other cytokines, leading to shock.

Examples:

• 
  
  Toxic shock syndrome toxin-1 (TSST-1) which is produced by staph.aureus.
  
  
  
• 
  
  Erythrogenic toxin which is produced by Streptococcus pyogenes.