L33: Microbial Pathogenicity

• Koch's Postulates

  a pathogen must be present in every individual with the disease
• Koch's Postulates

  A sample of the microorganism taken from the diseased host can be grown in pure culture
• Koch's Postulates

  A sample of the pure culture causes the same disease when injected into a healthy host
• Koch's Postulates

  The microorganism can be recovered from the experimentally infected host
• Process

  The suspected agent must be present in every case of the disease. The agent must be isolated and grown in pure culture. The cultured agent must cause the disease when it is inoculated into a healthy host.
• Key stages of microbial pathogenesis

  1. Adherence to host cells
  2. Invasion of host issues
  3. Replication within host cells
  4. Disease causing damage to host tissues
• Endotoxins are Lipopolysaccharides (LPS) components found in the outer membrane of gram-negative bacteria, and elicit strong immune responses
• Potential effects of endotoxic lipid A:

  • fever
  • inflammation
  • blood clotting
  • shock
• Exotoxins are proteins within living bacteria and the released into the surrounding medium. The 3 types are: Cytotoxins, Neurotoxins and Enterotoxins
• Cytotoxins are produced by the bacteria Streptococcus pyogenes causes the complete lysis of red blood cells
• Neurotoxins is produced by the bacteria Clostridium botulinum causes paralysis
• Enterotoxins (shiga toxin) produced by the bacteria Shigella dysenteriae causes severe dysentery
• Adherence to host cells

  adhesions such as fimbriae bind to host cells
• Invasion to host cells

  • motility (move through mucus) e.g flagella
  • internalin-related proteins
• Replication within host tissues

  • Siderophores bind iron. E.g it solubilises metal bound to host proteins and transports it back to the bacteria
• Disease causing damage to host tissues (pathology)

  • endotoxins
  • exotoxins