TOXINS.

Exotoxins.
Exotoxins are secreted products, usually of bacteria but sometimes protozoa and fungi. They can act in a number of ways:

• Inhibition of protein synthesis. Corynebacterium diphtheriae, the cause of diphtheria, produces a toxin that causes ADP-ribosylation of elongation factor-2, thereby stopping protein synthesis. It is extremely potent and one molecule of toxin is capable of killing a cell. Escherichia coli, Vibrio cholerae and Bordatella pertussis (the cause of whooping cough) also produce toxins that cause ADPribosylation of proteins.
Toxins from Shigella dysenteriae and E. coli strain O157:H7 (a cause of dangerous food poisoning) inhibit protein synthesis by removing adenine from 28s rRNA.
• Increase in cAMP. A number of bacteria produce toxins that raise cAMP levels. These include V. cholera, Bacillus anthracis, B. pertussis and some strains of E. coli. The consequence of increased cAMP levels is alterations in ion transport and hence fluid movement, often resulting in severe oedema.
• Neurotoxins. Members of the Clostridium family produce particularly potent neurotoxins. Clostridium tetani produces a toxin that prevents the release of glycine, an inhibitory neurotransmitter. This results in overactivity and muscle spasm including the typical lockjaw. Clostridium botulinum produces a neurotoxin that stimulates release of acetyl choline, leading to paralysis. It is one of the most potent toxins known and it is estimated that less than 1μg can kill a person.
• Enzymes that disrupt cell walls. Clostridium perfringens, a cause of gas gangrene, produces a toxin called α-toxin, which is a phospholipase that hydrolyses lecithin in the cell membrane, resulting in cell death.
• Superantigens. Some bacteria, particularly Staphylococcus and Streptococcus, produce toxins that cause excessive stimulation of the immune system (specifically of T lymphocytes; see Chapter 6). This leads to the production of factors by the immune system that cause the symptoms of shock. Toxic shock syndrome and food poisoning are two consequences of these toxins.