Diphtheria, botulism and tetanus

• Describe the two main types of A-B toxin

  Simple A-B toxins: consist of one A subunit and one B subunit, linked via a disulphide bond.

  Compound A-B toxin: variable number of A and B subunits.
• Describe the common action of A-B toxins

  A-B toxins removed the ADP ribosyl group from NAD and ADP-ribosylate target host protein, which usually leads to inactivation.
• Describe Diphtheria

  This disease is due to Corynebacterium Diptheriae, which is a gram negative facultative anaerobe. This bacteria only infects humans and is typically transmitted by inhalation of aerosols. First symptoms include tonsillitis, low grade fever and loss of appetite and a white pseudomembrane develops on the tonsils, soft palate and pharyngeal wall.
• Describe how Diptheria toxin expression is regulated

  High iron: iron binds to Diptheria toxin repressor (DtxR), which allows it to bind to the toxin operator and suppress transcription.

  Low iron: iron cannot bind to DtxR, so it can no longer bind to the toxin operator and therefore, transcription is not repressed.
• Describe the structure of diphtheria toxin

  This toxin consists of 3 functional regions:

  • A region: catalytic domain
  • T region: translocation domain
  • R region: receptor domain, binds to target cell

  The T and R regions make up the B subunit.
• Describe uptake of diphtheria toxin

  1. DT monomer binds to heparin binding epidermal growth factor (HB-EGF) receptor
  2. Receptor mediated endocytosis internalises the receptor bound DT
  3. pH decrease leads to formation of open DT monomer
  4. Insertion of T domain into the vesicle membrane, which leads to translocation of the A chain into the cytosol.
• Describe the action of A chain diphtheria toxin

  Diphtheria toxin ADP-ribosylates elongation factor 2, which inhibits protein synthesis.
• Describe botulism

  This disease is due to infection by Clostridium botulinum, which is a gram negative anaerobe and follows strictly fermentation metabolism. Disease occurs when spores germinate and grow in food. Produces botulinum toxins, which is a potent neurotoxin that induces nausea, headache, double vision and slurred speech.
• Describe the different types of food-born diphtheria

  • Foodborne botulism: C. botulinum in food
  • Infant botulism: C. botulinum spores ingested leading to C. botulinum in the GI tract
  • Wound botulism: C. botulinum spores in wound
• Describe structure and processing of botulinum toxins

  Progenitor toxin is cleaved by protease to form:

  • Heavy chain:
  • Consists of receptor binding domain and translocation domain
  • Light chain, enzymatic activity

  Both chains are linked by a disulphide bond
• Describe the action of botulinum toxins

  The cleaved toxin binds to neurons at the neuromuscular junction by binding to gangliosides and protein receptors. These receptors are then internalised by receptor mediated endocytosis. The A subunit exits the endosome and cleaves SNARE proteins, which prevents vesicle fusion with the membrane and prevents acetylcholine release and stops nerve impulse transmission.
• Describe the action of the different botulinum antigen serotypes

  1. B, D, F and G group: cleave synaptobrevin
  2. A and E group: cleave SNAP-25
  3. C group: cleaves SNAP-25 and syntaxin-1
• Describe the normal function of motor neurones

  1. Synaptobrevin binds to SNAP-25
  2. SNAP-25 complex binds to syntaxin
  3. The vesicle fuses with the plasma membrane
  4. Acetylcholine is released and binds to receptors on muscle cells leading to stimulation of the muscle cell and muscle contraction.
• What is tetanus?

  This disease is called by Clostridium tetani, which is a gram negative anaerobe that follows strictly fermentation metabolism. Disease occurs when spores germinate in wounds and produce potent neurotoxin, known as tetanus toxin. This is highly fatal and early symptoms include Painful spasms and rigidity of the voluntary muscles. Death usually occurs as a result of exhaustion and respiratory failure.
• Describe the action of tetanus toxin

  Tetanus toxin cleaves synaptobrevin and this prevents the release of inhibitory neurotransmitters, such as GABA and glycine at the inhibitor neurons of the spinal cord. This results in spastic paralysis.