week 10

• Infection


  Invasion of host tissues by a multiplying disease-causing microbe
• Pathogen


  Microbe capable of causing disease (includes opportunistic pathogens)
• Pathogenesis


  Process of disease - colonisation, evading immune system, spreading etc.
• Virulence


  Relative capacity of a microbe to cause damage in a host
• Virulence factor


  Molecules produced or strategies used by disease-causing microbes to enable them to colonise and cause disease
• Pathogenicity


  Ability or degree a microbe can cause damage in a host
• Attenuated


  Reduced virulence (natural or artificial)
• LD50


  Lethal dose 50% - the number of microbes that kill half the animals in an experimental group
• How to measure virulence: LD-50
• This depends on the pathogen
• How a pathogen differs from the normal microbiota

  • Avoid phagocytosis
  • Take on an undetectable latent stage
  • Efficiently slay their victim (Ebola)
  • Persists for years with minor symptoms
• Normal bacterial microflora of humans

  • Skin
  • Oral cavity
  • Respiratory tract
  • Gastrointestinal (GI) tract
  • Urogenital tract
• People living in a low-income country are far more likely to die of a communicable disease than a noncommunicable disease
• Obligate


  Can only grow and reproduce in host (e.g. Chlamydia trachomatis)
• Facultative


  Can cause disease but can grow and reproduce outside host (e.g. Vibrio cholerae)
• Opportunistic


  Can only cause infection when host is compromised (e.g. Clostridium difficile)
• Bacterial pathogens can be

  • Obligate
  • Facultative
  • Opportunistic
• All pathogens must


  • Enter a host
  • Find their unique niche
  • Avoid, circumvent, or subvert normal host defenses
  • Multiply
  • Be transmitted to a new susceptible host
• Virulence factors


  Molecules produced or strategies used by disease-causing microbes to enable them to colonise and cause disease
• Examples of virulence factors


  • Toxins
  • Attachment proteins
  • Capsules
  • Adhesins
  • Invasins
  • Impedins
• Stages of infection


  1. Entry

  2. Adhesion

  3. Invasion

  4. Evasion

  5. Transmission
• Bacteria must attach to cells to establish themselves - use adhesins
• Once attached, bacteria can produce molecules to help them spread and invade host tissue
• Invasion factors break down tissue structure
• Invasion factors attack cells
• Bacteria have strategies to evade the immune system
• Examples of mechanisms of defence against a pathogen in the host

  • Physical defence
  • Antibacterial factors
  • Excretory
  • Mucosal surfaces
  • Innate immune system
• Infection cycle


  The route of transmission an organism takes
• Transmission routes


  • Contact (direct, indirect)
  • Droplet
  • Vehicle
  • Airborne
  • Waterborne
  • Foodborne
  • Vector (mechanical, biological)
• Extreme pathogenicity is selected against
• Bacteria must have an opportunity to spread
• Balanced pathogenicity
• Koch's postulates


  • Pathogen must be present in every disease case
  • Pathogen must be able to be isolated and cultured from disease cases
  • Cultured isolate must cause disease when inoculated into a host
  • The same pathogen must be re-isolated from infected host
• Problems with Koch's postulates
• Molecular Koch's postulates


  Modern application based on identification of copy numbers of sequence
• Sources of infection


  • Animal reservoirs (zoonoses)
  • Human carriers
  • Non-living reservoirs (soil, water)
• Stages in the disease process

  • Endogenous infections
  • Exogenous infections
  • Nosocomial (hospital) infections
• Antibiotics


  • Aminoglycosides
  • β-lactams
  • Macrolides
  • Quinolones
  • Sulfonamides
  • Tetracyclines
• Antibiotic resistance is becoming widespread
• Mechanisms of antibiotic resistance

  • Aminoglycosides
  • β-lactams
  • Macrolides
  • Quinolones
  • Sulfonamides
  • Tetracyclines