Bacterial pathogenicity

• Pathogenicity

  The ability to cause disease
• Virulence

  The degree of pathogenicity (determined by toxicity and invasiveness)
• Host

  Organism which supports growth of viruses, bacteria and parasites
• Pathogen

  Organism that causes disease, by impairing or interfering with the normal physiological activities of the host
• Infection

  bacteria persist in host without necessarily causing tissue damage
• Disease

  overt damage to the host, parts of body cannot fulfill their normal functions
• Successful bacterial pathogens

  • Opportunistic pathogens
  • Primary pathogens (Obligate)
• Opportunistic pathogens

  Only cause disease when immunity is weakened
• Primary pathogens (Obligate)

  Capable of causing disease even in a host with a strong immune system.

  They need to cause disease to survive.
• Steps in infectious disease

  1. Reservoir

  2. Transport to host

  3. Adherence & Colonisation

  4. Invasion of tissues

  5. Tissue damage
• Reservoirs for bacterial pathogens

  • Other humans
  • Animals
  • Environment
• What are the two classifications of transfer of a pathogen to host

  Direct or indirect
• Colonisation

  Establishment of a stable population of bacteria in the host
• Adherence

  Bacteria must adhere to host cell surfaces to overcome flushing mechanisms
• Adherence process

  • 1st stage: Association (non-specific forces)
  • 2nd stage: Adhesion (specific bacterial adhesins and host receptors)
  • May result in aggregation to produce a biofilm
• Describe iron uptake by bacteria

  Bacteria express a high affinity for iron uptake systems in the host because the free iron levels are below what bacteria require for growth.

  They can either use siderophores to bind iron or directly bind iron transport proteins such as transferrin.
• Invasion

  Bacteria penetrating into, through or between host cells and tissues
• Describe thee use Lytic compoundsfor bacterial invasion

  Compounds that attack and damage host tissue
• Invasins

  Bacterial virulence factors that enable invasion
• Host defences

  • Macrophages and other phagocytes engulf and kill bacteria
  • Cytotoxic cells kill cells infected with bacteria
• Bacterial avoidance of phagocytosis

  • Resisting phagocytosis by producing structures like capsules and surface proteins
  • Survival inside phagocytic cells, often by very pathogenic bacteria
• Antibody avoidance mechanisms


  • Capsules (sometimes not immunogenic)
  • Antigenic variation
  • Degradation of antibodies
• Tissue damage


  May occur through direct effects of bacterial toxins, indirect effects of bacterial toxins, or induction of autoimmune responses
• Bacterial toxins


  • Exotoxins
  • Endotoxin (LPS)
• Exotoxins


  • Protein toxins secreted by living bacteria, highly immunogenic, potentially lethal
• Endotoxin (LPS)


  • Lipopolysaccharide bound to cell, released on cell lysis, weakly immunogenic, lethal at higher concentrations
• Human exposure to exotoxins


  • Ingestion of preformed exotoxin (food poisoning)
  • Colonisation of mucosal surface or tissue followed by toxin production (cholera)
  • Colonisation of wound followed by toxin production (gas gangrene)
• Structure of LPS (endotoxin)


  Lipid A part can have endotoxin activity, activates many host systems that cause damage
• Prokaryotes are not 'primitive' organisms - highly evolved though 3.5 billion years
• Prokaryotes' simplicity and adaptability is part of their beauty
• Prokaryotic structures and adaptations


  • Circular chromosome with plasmids
  • Fimbria, pili, flagella
  • Capsule
  • No nucleus or membrane enclosed organelles
• Rapid reproduction, mutation, and genetic recombination contribute to prokaryotic success
• Prokaryotes have diverse nutritional and metabolic adaptations, which can be beneficial or harmful (mutualist, commensal, pathogen)
• What are the main methods of direct transmission of pathogens

  Airborne or bodily contact
• What are the main classifications of indirect pathogen transfer

  Vector -borne or vehicles.
• What is vehicular pathogen transport


  The use of non-living objects.
• What is vector-borne pathogen transport

  The use of a living organism such as arthropods or vertebrates.
• What type of orgnaism are insect and ticks

  Anthropods
• How does airbone transmission occur

  Through aerosols such as coughing or sneezing