Bacterial pathogenicity 1

• pathogen life cycle

  reservoir - transmission - colonisation - invasion - proliferation - transmission

  
• pathogenicity


  ability of an organism to cause disease
• virulence


  degree of harm caused by a microorganism
• pathogenicity factors


  virulence, number of bacteria and host resistance
• virulence factors


  - adhesion

  - invasion

  - evasion of host defence

  - obtaining nutrients from host

  - toxicity
• measuring virulence


  - infectious dose = ID50: dose to infect 50% hosts

  - lethal dose = LD50: dose to kill 50% hosts
• 2 entry portals

  1. skin = only if barrier's damaged (wounds, bites)

  2. mucosal surfaces = more favourable - warm, moist, more nutrients
• direct transmission routes


  1. respiratory = coughing/sneezing droplets (TB, influenza)

  2. body contact = STDs, skin infections (ringworm, warts)

  3. faecal/oral = through contact (food) or GIT pathogens (salmonella)

  4. body fluids = hepatitis, HIV

  5. vertical = prenatal/perinatal/postnatal, germline (vDNA e.g. leukaemia)
• indirect vehicles


  1. soil (clostridium perfringens)

  2. contaminated water (cholera)

  3. food (lysteria)

  4. fomites (c.diff)

  5. vectors = mosquitoes (malaria), warm-blooded mammals (rabies), rat fleas (black plague)
• bacterial adhesion


  must adhere to host cells/tissues to colonise

  depend on ADHESINS
• adhesins


  - interact with receptors on host's cell surface (protein-protein, protein-carb)

  - receptors can be membrane proteins, glycolipids, extracellular matrix proteins (collagen etc)

  1. proteins = fimbrial, other surface proteins (pilli+flagellum)

  2. polysaccharides = capsule components, teichoic and lipoteichoic acid
• 2 invasion types


  1. extracellular = barriers of tissues broken down

  2. intracellular = microbes penetrate cells and survive intracellularly
• extracellular invasion


  • Allows access to niches in tissue that aid in proliferation and spreading

  •Achieved through production of enzymes that:

  1. attack extracellular matrix

  2. degrade carbohydrate-protein complexes between cells

  3. disrupt cell surface
• phagocytosis


  1. WBC meets bacterium that binds to membrane

  2. phagocyte uses cytoskeleton to push membrane around bacterium - creating a phagosome (vesicle)

  3. phagosome separates from membrane into cytoplasm

  4. phagosome fuses with lysosomes containing digestive enzymes

  5. bacterium killed and digested within vesicle

  
• surviving phagocytosis


  - reside in phagolysosome

  - reside in unfused phagosome (salmonella)

  - destroy or escape phagosome and live in cytosol
• invading non-phagocytic cells

  - bacterial proteins recruit host proteins to induce phagocytosis (e.g. secretion system used by some G-ve

  - invasion proteins injected - activate host signalling and recruit actin