Bacterial Toxins

Pathogenic bacteria cause disease in two main ways: by invading and damaging host tissues, and by producing toxins. A toxin is a poisonous substance that damages host cells or disrupts the way the body works. Many of the symptoms of bacterial disease, such as fever, diarrhoea and vomiting, are caused by toxins rather than by the bacteria themselves.Bacterial toxins fall into two groups.Endotoxins are lipopolysaccharides (lipid joined to carbohydrate) that form part of the outer layer of the cell wall of Gram-negative bacteria. They are released mainly when the bacteria die and break open. Salmonella is an example.Exotoxins are soluble proteins that living bacteria make and secrete into their surroundings. They are produced mainly by Gram-positive bacteria, such as Staphylococcus, though some Gram-negative bacteria make them too.The two groups differ in several ways:
• Source: endotoxins are part of the cell; exotoxins are released by living cells.
• Chemical nature: endotoxins are lipopolysaccharide; exotoxins are protein.
• Heat: endotoxins are relatively heat-stable; most exotoxins are denatured by heat.
• Effects: endotoxins cause general effects such as fever and inflammation; exotoxins usually have specific effects on particular cells or tissues, often at very low doses.A toxin can act far from the site of infection, because it can be carried around the body in the blood. The next lessons look at each type in detail, with Salmonella as an example of endotoxins and Staphylococcus as an example of exotoxins.