Exotoxins and endotoxins of bacterial toxins
Bacterial toxins are of two types and they are –
Exotoxins and
Endotoxins.
1. Exotoxins
Exotoxins of bacteria toxins are heat-labile proteins that are secreted by certain species of bacteria. They diffuse readily into the surrounding medium. They are highly potent even in small amounts and constitute some of the most poisonous substances known. Botulinum toxin is the most poisonous, it has been estimated that 3 kg of botulinum toxin can kill all the inhabitants of the world. Exotoxins can be converted into toxoids by treatment with formaldehyde. Toxoids lack toxicity but retain antigenicity and therefore induce protective immunity when used as vaccines. Exotoxins are highly antigenic and stimulate the formation of antitoxin which neutralizes toxins. They are highly specific for a particular tissue e.g. tetanus toxin for the central nervous system. They have specific pharmacological activities and do not produce fever in the host. They are mainly produced by gram-positive bacteria but may also be produced by certain gram-negative bacteria e.g. Vibrio cholerae and enterotoxigenic E. coli.
2. Endotoxins
Endotoxins of bacterial toxins are lipopolysaccharide in nature and form an integral part of the cell wall of gram-negative bacteria. They are released from the bacterial surface by the natural lysis of the bacteria or by the disintegration of the cell wall. They are heat stable. The toxigenicity of endotoxins depends on the lipid component (lipid A). Lipopolysaccharide binds to neutrophils via lipid A and alters neutrophil functions as follows:
- It decreases chemotactic activity.
- Random migration of neutrophils is decreased.
- It alters the metabolic and bactericidal properties of neutrophils.
The endotoxins are not destroyed by autoclaving, hence infusion of a sterile solution containing endotoxins can cause serious illness. They can not be toxoided. They are poor antigens and the toxicity is not completely neutralized by the homologous antibodies. Massive gram-negative septicemias may cause the syndrome of endotoxic shock characterized by fever, leucopenia, thrombocytopenia, profound fall of blood pressure, and circulatory collapse leading to death.
