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An exotoxin is a soluble chemical excreted by a microorganism, including bacteria, fungi, algae, and protozoa. An exotoxin is a protein which can cause damage to the host 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, similar to endotoxins, may be released during lysis of the cell.

Most exotoxins can be destroyed by heating. They may exert their effect locally or produce systemic effects. (Nester, 2007). Well known exotoxins include the botulinum toxin produced by Clostridium botulinum, the Corynebacterium diphtheriae exotoxin which is produced during life threatening symptoms of diphtheria.

Exotoxins are susceptible to antibodies produced by the immune system, but many exotoxins are so toxic that they may be fatal to the host before the immune system has a chance to mount defenses against it (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 and endopeptidase.

• A-B type toxins - the 'B'-subunit attaches to target regions on cell membranes, the 'A'-subunit enters through the membrane and tends to possess enzymatic function that affect internal cellular bio-mechanisms. The structure of these toxins allows for the development specific vaccines and treatments. Certain compounds can be attached to the B unit, which is not generally harmful, which the body learns to recognize, and which elicits an immune response. This allows the body to detect the harmful toxin if it is encountered later, and to eliminate it before it can cause harm to the host. 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 Clostridium perfringens. Exotoxins that act on red blood cells and cause hemolysis.

• Infectious disease
• Endotoxin

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