Listeria monocytogenes

• Describe listeria monocytogenes

  • Gram positive, rod shaped, facultative anaerobe
  • Ubiquitous within plants, soil, water and animals
  • First conclusive evidence of foodborne transmission of L. monocytogenes was discovered during a large outbreak of foodborne listeriosis in Canada 1981.
• What is listeriosis?

  This is a rare but potentially life-threatening disease that usually occurs in immunocompromised individuals. This leads to influenza-like symptoms due to bacteraemia and, in pregnant women, neonatal infection, meningitis and abortion. It is an uncommon disease but exhibits a high mortality rate of around 25-30%.
• Describe listeria as a foodborne pathogen

  Listeria grows at low temperatures, including refrigeration temperatures of below 5 degrees C. They are also resistant to high salt and acid, which is problematic since these are food preservative techniques. They can be killed by cooking and pasteurisation. Foods most likely to be contaminated include unpasteurised cheeses, cold cuts of meat and smoked fish.
• Describe the life cycle of listeria monocytogenes

  1. Entry via binding of attachment proteins known as internalin, lnA/B.
  2. Lysis of the vacuole using listeriolysin O (LLO)
  3. This allows the bacteria to move intracellularly
  4. Cells can then spread into another cell by pushing against the membrane, without becoming exposed to the extracellular environment
  5. This leads to formation of a double membrane, which can be lysed and the bacteria escape.
• Describe the internalin genes in L. monocytogenes

  • 25 internalin genes in the L monocytogenes genome
  • 19, including lnlA, covalently linked to bacterial cell wall
  • only lnlB has C terminal GW repeats involved in loose attachment to bacterial lipoteichoic acid
  • lnlC is secreted due to no anchoring signal
• Describe the function of internalin

  lnlA and lnlB bind to different host proteins to promote receptor mediated endocytosis. LnlA binds to E-cadherin of non-Phagocytic cels, promoting receptor clustering, phosphorylation, ubiquitination and bacterial uptake. LnlB binds to hepatocyte growth factor receptor and induces phosphorylation, ubiquitination and receptor mediated endocytosis. Soluble released lnlB binds to complement proteins and to cell surface glycosaminoglycans to promote receptor clustering and enhanced bacterial uptake.
• Describe how L. monocytogenes escape from the phagosome

  This is mediated by Listeriolysin O, which is a member of the pore-forming cholesterol dependent cytolysins. Sterols such as cholesterol not found in bacterial membranes.
• Describe structure and pore formation by LLO

  LLO belongs to the family of cholesterol-dependent cytolysins (CDCs). They bind to cholesterol in host cell membranes and then diffuse laterally to form a ring-shaped oligomeric pre-pore complex. This complex then inserts a large beta-barrel pore across the membrane That is around 35nm wide.
• Describe the function of ActA

  This is an integral membrane protein that promotes actin polymerisation and is required for actin based motility, cell-to-cell spread and rapid dissemination of the bacteria in the infected host. ActA is produced as a single pole of the cells and directly interacts with, and activates, the Arp2/3 complex.
• Describe listeria virulence gene regulation

  Most of the virulence proteins are under control of a single transcriptional activator, known as positive regulatory factor A (prfA). This regulator binds to a 14bp palindromic sequence located within the promoter regions And stimulates transcription. Examples of genes under prfA regulation:

  • listeriolysin O
  • Internalin A, B and C
• Describe thermoregulation of PrfA translation

  Low temp (30 degrees C): secondary structure forms in the utr region, which prevents ribosome binding and therefore expression of PrfA

  High temp (37 degrees C) melting of the prfA UTR leads to ribosome binding and PrfA expression.