Exotoxin
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An exotoxin is a soluble chemical excreted by a microorganism, including bacteria, fungi, algae, and protozoa. This is a type of toxin that is produced by bacteria and released into the bloodstream. An exotoxin is a protein which can cause damage in the body by destroying cells or disrupting normal cellular metabolism. Both gram negative and gram positive bacteria produce exotoxins. They are highly potent and can cause major damage to the host. Exotoxins may be secreted, or released during lysis of the cell (similar to endotoxins). Most exotoxins can be destroyed by heating. They may exert their effect locally or produce systemic effects. (Nester, 2007). Examples include the botulinum toxin produced by Clostridium botulinum, the Corynebacterium diptheriae exotoxin which is produced during life threatening symptoms of diptheria. Being proteins, exotoxins are potentially susceptible to antibodies produced by the immune system, but many exotoxins are so dangerous they can be fatal before the immune system even has a chance to mount a defense. This is why immunisations agains tetanus and diptheria are beneficial. (Nester, 2007).
There are three main types of exotoxins:
- toxins that act upon the extracellular matrix or connective tissue - this allows the further spread of bacteria and therefore the infection deeper into tissue regions.
- Examples are protease, endopeptidase.
- A-B type toxins - the 'B'-subunit attaches to target regions on cell membranes, the 'A'-subunit tends to have an enzymatic function and enters through the membrane affecting the internal bio-mechanisms. The structure of these toxins allows for specific vaccines and treatments. Certain compounds can be attached to the B unit to elicit the desired immune response. The body learns to recognize the B unit which is not harmful, and prepares the body for future exposure to the real toxin.
- Toxins of this type include cholera toxin, pertussis toxin, shiga toxin and heat-labile enterotoxin from E. coli.
- Membrane damaging toxins - these toxins are designed primarily to puncture and disrupt the cellular membrane killing cells, a significant resulting example is that of Gangrene caused by Clostridum perfringens. Exotoxins that act on RBCs cause hemolysis.
[edit] See also
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