Enterotoxin and Exotoxin
Enterotoxin and exotoxin are two classes of toxin that are produced by bacteria.
An exotoxin is a toxin that is produced by a bacterium and then released from the cell into the surrounding environment. The damage caused by an exotoxin can only occur upon release. As a general rule, enterotoxins tend to be produced by Gram-positive bacteria rather than by Gram-negative bacteria. There are exceptions, such as the potent enterotoxin produced by Vibrio cholerae. In contrast to Gram-positive bacteria, many Gram-negative species posses a molecule called lipopolysaccharide. A portion of the lipopolysaccharide, called the lipid A, is a cell-associated toxin, or an endotoxin.
An enterotoxin is a type of exotoxin that acts on the intestinal wall. Another type of exotoxin is a neurotoxin. This type of toxin disrupts nerve cells.
Many kinds of bacterial enterotoxins and exotoxins exist. For example, an exotoxin produced by Staphylococcus aureus is the cause of toxic shock syndrome, which can produce symptoms ranging from nausea, fever and sore throat, to collapse of the central nervous and circulatory systems. As another example, Staphylococcus aureus also produces enterotoxin B, which is associated with food-borne illness. Growth of the bacteria in improperly handled foods leads to the excretion of the enterotoxin. Ingestion of the toxin-contaminated food produces fever, chills, headache, chest pain and a persistent cough. This type of illness is known as a food intoxication, to distinguish it from bacterial food-borne illness that results from growth of the bacteria following ingestion of the food (food poisoning).
Enterotoxins have three different basis of activity. One type of enterotoxin, which is exemplified by diphtheria toxin, causes the destruction of the host cell to which it binds. Typically, the binding of the toxin causes the formation of a hole, or pore, in the host cell membrane. Another example of a pore-forming exotoxin is the aerolysin produced by Aeromonas hydrophila.
A second type of enterotoxin is known as a superantigen toxin. Superantigen exotoxins work by overstimulating the immune response, particularly with respect to the T-cells. Examples of superantigen exotoxins include that from Staphylococcus aureus and from the "flesh-eating" bacterium Streptococcus pyogenes.
A third type of enterotoxin is known as an A-B toxin. An A-B toxin consists of two or more toxin subunits that work together as a team to exert their destructive effect. Typically, the A subunit binds to the host cell wall and forms a channel through the membrane. The channel allows the B subunit to get into the cell. An example of an A-B toxin is the enterotoxin that is produced by Vibrio cholerae.
The cholera toxin disrupts the ionic balance of the host's intestinal cell membranes. As a result, the cells of the small intestine exude a large amount of water into the intestine. Dehydration results, which can be lethal if not treated.
In contrast to the destructive effect of some exotoxins, the A-B exotoxin (an enterotoxin of Vibrio cholerae does not damage the structure of the affected host cells. Therefore, in the case of the cholera toxin, treatment can led to a full resumption of host cell activity.
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