Interaction of pathogens (with a host)

Interaction of pathogens (with a host)

Intended learning outcomes

  • Describe how pathogens cause disease and how they overcome the innate host defence
  • Demonstrate an understanding of basic concepts of epidemiology: how infections are propagated.

Commensal bacteria:

  • Commensal bacteria act on the host's immune system to induce protective responses that prevent colonization and invasion by pathogens.
    • Aid immune development
    • Colonisation resistance
    • Metabolic organ
  • But can cause opportunistic infection in immunocompromised host - known as opportunistic pathogens

What is a immunocompromised host?

  • Damage to epithelial such as skin infections (bacteria can't get through skin if its is intact)
  • Disruption of commensal bacteria
  • Impairment of the immune system caused by infection, chemotherapy or age
  • Introduction of bacteria into new sites such as device associated infection

Obligate human pathogens:

  • Disseminate solely through causing disease in humans
  • Not part of the commensal flora, they have a poor survival rate outside the host

How do bacteria spread:

  • Formite - inanimate object (e.g phone)
  • Vector - blood feeding ( e.g mosquitos)
  • Vehicle - food or water

How do bacteria cause infections:

  • Bacteria have virulence factors which can cause infection
    • Virulence factors are bacterial products or strategy that contributes to virulence or pathogenicity.
  • Main categories of virulence factors:
    • Promote colonisation and invasion
    • Damage host

Colonisation and invasion:

  • Fimbria/pilli
    • Proteinaceous, non-flagella surface appendages
    • Anchored to outer membrane enable the bacteria to adhere to host
    • Different pilli have different targets
  • Flagella
    • Thin hollow, appendages which allow the bacteria to move via rotation
  • Capsules
    • A polysaccharide layer which surround the cell , provides attachment and prevents being stuck in mucus
    • Capsule also helps the bacteria avoid immune invasion to cause a systemic infection.
    • Bacteria avoid immune response by mimicking the host cell and preventing phagocytosis
  • N.meningitidis
    • Capsule has such a massive negatively charge that anything else that has a negative charge will be pushed away (phagocyte will be pushed away due to repulsion due to negative charge allowing to avoid phagocytosis)
    • Prevents immunogenic response as it mimic host, so doesn't trigger immune response
    • uncontrolled growth in the blood leads to meningococcal septicaemia

Endotoxins:

  • Part of gram negative bacteria
  • Part of lipopolysaccharide layer of the cell wall
  • When bacteria die it cause endotoxins to be released which will enter surrounding area which trigger cytokine release

Exotoxins:

  • Bacterial metabolites are actively excreted from the focus of the infection to specific parts of the body

Pneumolysin and CAP

  • Pneumolysin is an pore forming exotoxin which is produced by S.pneumoniae, it is critical in the development of community acquired pneumonia
  • pneumolysin causes cell apoptosis to macrophages
  • For an immune response the body produces a proinflammatory reaction which is:
    • Characterised by influx of neutrophils
    • Complement activation of reactive oxygen species
  • Virulence factors can allow the bacteria to avoid this reaction which can cause inflammatory mediated damage
  • Very young and the elderly are more susceptible to CAP

Signs and symptoms of CAP:

  • Fever
  • Shortness of breath
  • Productive cough
  • Focal chest signs