Exotoxins are soluble proteins made by living bacteria and secreted into their surroundings. They are produced mainly by Gram-positive bacteria, such as Staphylococcus, Clostridium and the bacterium that causes diphtheria, but some Gram-negative bacteria, such as Vibrio cholerae, also produce them.Because they are released into tissue fluid and blood, exotoxins can act far from the bacteria that made them. Each exotoxin has a specific target, and exotoxins are among the most poisonous substances known: a tiny mass can be lethal.Exotoxins work in different ways, for example:
• damaging cell membranes: some form pores in the plasma membranes of host cells, so the cells leak and burst (lysis);
• interfering with nerve impulses: botulinum toxin blocks the release of neurotransmitter at synapses, causing paralysis;
• altering cell processes: cholera toxin makes gut epithelial cells secrete chloride ions into the gut, water follows by osmosis and severe diarrhoea results;
• overstimulating the immune system: some act as superantigens, causing a massive release of cytokines.Key properties:
• they are strongly antigenic, so the immune system makes antibodies (antitoxins) against them;
• most are heat-labile: heating denatures the protein and destroys its activity;
• they can be treated with heat or chemicals to make toxoids, harmless forms that keep their antigens. Toxoids are used as vaccines, for example against tetanus and diphtheria.Some exotoxins, including some made by Staphylococcus, are unusually heat-stable, so reheating contaminated food does not always make it safe.
• damaging cell membranes: some form pores in the plasma membranes of host cells, so the cells leak and burst (lysis);
• interfering with nerve impulses: botulinum toxin blocks the release of neurotransmitter at synapses, causing paralysis;
• altering cell processes: cholera toxin makes gut epithelial cells secrete chloride ions into the gut, water follows by osmosis and severe diarrhoea results;
• overstimulating the immune system: some act as superantigens, causing a massive release of cytokines.Key properties:
• they are strongly antigenic, so the immune system makes antibodies (antitoxins) against them;
• most are heat-labile: heating denatures the protein and destroys its activity;
• they can be treated with heat or chemicals to make toxoids, harmless forms that keep their antigens. Toxoids are used as vaccines, for example against tetanus and diphtheria.Some exotoxins, including some made by Staphylococcus, are unusually heat-stable, so reheating contaminated food does not always make it safe.