Brief notes on bacterial toxins. Many bacteria produce toxins which harm the host and enable them to spread thus causing infections. The term toxaemia refers to the presence of toxins in the blood and the ability of bacteria to produce toxins is called toxigenicity. Toxin production is linked to an organism's pathogenicity (along with ability to adhere to the host and evade the immune attack). Exotoxins (eg alpha-haemolysin, PVL, proteinase, DNAse, hyaluronidase) are produced by bacteria and released into the surrounding medium (eg host tissue). Exotoxins, not the bacteria, produce the disease symptoms. Antibodies produced against exotoxins are called antitoxins. Bacterial toxins may be named according to what type of cells they attack (neurotoxins, cardiotoxins, hepatotoxins, leukotoxins, enterotoxins, cytotoxins) the diseases they are associated with (eg. toxic shock toxin 1 (TSST1), tetanus toxin, exfoliative toxin) or for the bacterium that produces them (botulinum toxin, Vibrio enterotoxin). There are three basic types of exotoxin based on structure and function: · Type III (intracellular) toxins (A-B toxins) · Type II (Membrane disrupting) toxins · Type I (Cell surface active) toxins (also called Superantigens) Type III -A-B Toxins Most exotoxins are A-B or type III toxins. A-B toxins consist of two polypeptides. The A polypeptide is the enzyme and the B polypeptide is a binding component. B binds to the cell, the complete toxin is taken into the cell, the two subunits separate, and the A subunit enzymatically kills the cell. Type II - Membrane Disrupting Toxins Type II toxins, disrupt host cell plasma membranes. Leukocidins kill phagocytes Haemolysins destroy erythrocytes Streptolysins are haemolysins produced by Streptococci Type I - Cell surface active toxinsSuperantigens Type I toxins, provoke intense immune responses.
Representative Exotoxins Cytotoxins i. Diphtheria toxin (A-B toxin which inhibits protein synthesis) - Corynebacterium diphtheriae ii. Erythrogenic toxins (superantigens that damage capillaries) - Streptococcus pyogenes Neurotoxins i. Botulinum toxin (A-B toxin which prevents nerve impulse transmission from motor neuron to muscle fiber at motor endplate, results in flaccid paralysis) - Clostridium botulinum ii. Tetanus toxin, or tetanospasmin (A-B toxin which prevents inhibitory nerve transmission, results in spasmodic muscle contractions) - Clostridium tetani Enterotoxins - induce fluid and electrolyte loss from host cell i. Vibrio entertoxin - A-B toxin produced by Vibrio cholerae, stimulates secretion of fluid and electrotytes by mucosal epithelial cells, results in diarrhea. toxin ii. Staphylococcus aureus - staphylococcal enterotoxin, a superantigen that produces the same results as Vibrio enterotoxin. Endotoxins and Exotoxins
Cell wall Exotoxin (a) Exotoxins are proteins produced inside pathogenic bacteria, most commonly gram-positive bacteria, as part of their growth and metabolism. The exotoxins are then secreted or released into the surrounding medium following lysis. Copyright @ 2010 Pearson Education, Inc. Endotoxin (b) Endotoxins are the lipid portions of lipopolysaccharides (LPSs) that are part of the outer membrane of the cell wall of gram-negative bacteria (lipid A; see Figure 4.13c). The endotoxins are liberated when the bacteria die and the cell wall breaks apart. Table 15.2 Diseases Caused by Exotoxins Disease Bacterium Botulism Clostridium botulinum A-B Tetanus