Bacteria and Viruses 2

• Endogenous bacterial infection


  When commensal bacteria infect another part of the human body
• Exogenous bacterial infection


  External pathogenic bacteria infect the human body
• Exogenous bacterial infections


  • TB
  • Scarlet Fever
  • Syphilis
• Endogenous bacterial infections


  • Acne
  • Pimples
  • Boils
  • UTIs
• Factors required for bacterial growth
• Portals of entry for infectious bacterial agents

  • Ingestion
  • Inhalation
  • Direct Penetration
  • Colonisation
  • Invasion
  • Multiplication
• Pathogenicity


  The quality of producing disease or the ability to produce pathologic changes or disease
• Virulence


  A measure of pathogenicity; one general standard is the LD50 (lethal dose 50%)
• Infection


  The colonization and/or invasion and multiplication of pathogenic microorganisms in the host with or without the manifestation of disease
• Disease


  An abnormal condition of body function(s) or structure that is considered to be harmful to the affected individual (host)
• Koch's 4 Postulates for identifying infectious disease
• Bacterial virulence factors


  • Adherence (Colonization)
  • Invasion
  • Degradative enzymes
  • Exotoxins
  • Endotoxin
  • Induction of excess inflammation
  • Evasion of Immune System
  • By-products of growth (gas, acid)
  • Superantigen
  • Resistance to antibiotics
• Endotoxins


  Part of the cell wall of Gram-negative bacteria, causes fever, leukopenia, metabolic effects, lymphocyte factors, cellular death
• Exotoxins


  Secreted by bacteria into surroundings, A-B toxins with binding domain and enzymatic domain with intracellular targets
• Differences between endotoxins and exotoxins
• Bacterial virulence mechanisms for immune evasion

  • Encapsulation
  • Antigenic Mimicry
  • Masking
  • Shift
  • Evasion or incapacitation of Phagocytosis
  • Resistance to Humoral Factors
  • Resistance to Cellular Factors
• Bacterial virulence factors can cause direct or indirect host damage
• Factors influencing viral disease outcome
• Pathogenicity


  The ability of a microorganism to cause disease
• Virulence


  The degree of pathogenicity of a microorganism
• Infection


  The invasion and multiplication of microorganisms in body tissues
• Disease


  A disorder of structure or function in a human, animal, or plant, especially one that produces specific signs or symptoms or that affects a specific location and is not simply a direct result of physical injury
• Viruses are obligate parasites so they must arise from a current host of the virus
• Viruses don't survive in the environment for very long at all. Most are sensitive to sunlight (UV) and elevated temperatures
• Viral tropism


  Viruses target and replicate in particular cells, tissues or an organ
• Factors influencing viral tropism

  • Cell transcription factors that recognize viral promoters and enhancer sequences
  • Ability of the cell to support virus replication
  • Local temperature, pH, and oxygen tension enzymes and non-specific factors in body secretions
• Types of viral infection

  • Primary replication
  • Unapparent infection
  • Localized infection
  • Systemic (disseminated) infection
  • Secondary replication (tissue tropism)
• Incubation period


  The period of time between exposure to virus and onset of disease
• Types of viral infections

  • Acute infections
  • Persistent infections (acute with late complications, chronic, latent, slow, oncogenic)
• Latent infection


  Long term latency in lymphocytes - virus stimulates cell division and genome maintained as an episome
• Viruses infecting gut or respiratory tract spread rapidly in fluid bathing epithelial surface
• Localized viral infections can cause severe tissue damage or provide opportunity for secondary bacterial infection
• Viraemia must have high virus load and/or long duration for neurotropic viruses to spread from blood to brain
• Only a few blood-borne viruses can cross the placenta, causing severe effects like death, abortion, and teratogenic effects
• Routes of viral shedding

  • Respiratory secretions
  • Skin
  • Saliva
  • Faeces
  • Semen, milk, cervical secretions
• Antigenic shift


  Major, abrupt change in Influenza A viruses, resulting in a new subtype with no immunity
• Antigenic drift


  Small changes in influenza genes over time during replication, producing genetically related viruses with similar antigenic properties
• Targets for antiviral therapies

  • Virus attachment/penetration
  • DNA virus replication
  • Retrovirus reverse transcriptase
  • Protease inhibitors
  • Interferons