Pathogenesity

• Pathogenesis


  The origin and development of a disease

  
• Infection


  The growth of microorganisms that are not normally present within the host
• Host


  An organism that harbors a pathogen, another organism that lives on or in the host and causes disease
• Disease


  Tissue damage or injury that impairs host function
• Pathogenicity


  The ability of a microorganism to cause disease

  
• Opportunistic pathogen


  Causes disease only in the absence of normal host resistance

  
• Virulence


  The relative ability of a pathogen to cause disease

  
• Attenuation


  The decrease or loss of virulence of a pathogen
• Attenuation


  • Occurs because nonvirulent or weakly virulent mutants grow faster than virulent strains in laboratory media, where virulence has no selective advantage
  • Valuable in the production of vaccines
• Adherence


  The enhanced ability of a microorganism to attach to a cell or surface

  
• Glycocalyx


  A polymer secreted by a bacterium that coats the surface of the bacterium

  
• Glycocalyx


  • Capsules
  • Slime layers
• Adherence factors


  • Capsule of Bacillus anthracis
  • Slime layers of Vibrio cholerae
  • Polysaccharide capsule of Streptococcus pneumoniae

  
• Fimbriae, pili, flagella


  Microbial surface structures that facilitate adherence to host cells

  
• Adherence factors


  • Pili of Neisseria gonorrhoeae
  • Hemagglutinin of influenza virus
  • Fimbriae of Escherichia coli
  • Lipoteichoic acid, F protein, M protein of Streptococcus pyogenes

  
• Invasion


  The ability of a pathogen to enter into host cells or tissues, spread, and cause disease
• Bacteremia


  The presence of bacteria in the bloodstream
• Septicemia


  A bloodborne systemic infection
• Infection


  The biological process, which takes place in the body of organisms following the penetration of pathogenic microorganism

  
• Disease


  Damage or injury produced by microorganisms that impairs host function
• Infection requires growth of microorganisms after they have attached to surfaces
• Biofilm formation on tooth surfaces

  • Streptococcus sobrinus
  • Streptococcus mutans
• Dental plaque is a mixed-culture biofilm composed of several different genera and their accumulated products
• Dental plaque produces high concentrations of organic acids, particularly lactic acid, that cause decalcification of the tooth enamel resulting in dental caries (tooth decay)
• Virulence factors


  Factors that indirectly or directly enhance invasiveness by promoting pathogen infection
• Virulence factors


  • Hyaluronidase produced by streptococci, staphylococci, and clostridia
  • Collagenase produced by clostridia that cause gas gangrene
  • Proteases, nucleases, and lipases produced by pathogenic streptococci and staphylococci
  • Streptokinase produced by Streptococcus pyogenes
  • Coagulase produced by Staphylococcus aureus

  
• Pathogenicity islands


  Clusters of virulence genes found on the chromosome of some pathogens
• Pathogenicity islands in Salmonella

  • Salmonella pathogenicity island 1 (SPI1)
  • Salmonella pathogenicity island 2 (SPI2)

  
• Injectisome


  An organelle in the bacterial envelope that allows direct transfer of virulence proteins into host cells through a needle-like assembly

  
• Exotoxins

  Toxic proteins released from the pathogen as it grows, which travel from the site of infection and inhibit host cell function or kill host cells
• Salmonella pathogenicity island 1 (SPI1)


  A collection of virulence genes of Salmonella that encode at least 10 different proteins that promote invasion

  
• invH


  • Encodes a surface adhesion protein
  • Other inv genes encode proteins important for trafficking of virulence proteins
• Salmonella pathogenicity island 2 (SPI2)

  Contains genes that are responsible for causing more systemic disease and resistance to host cell defenses

  
• Virulence


  The ability of an organism to cause disease by means of a preformed toxin that inhibits host cell function or kills host cells

  
• Exotoxins


  Toxic proteins released from the pathogen as it grows, that travel from a site of infection and cause damage at distant sites

  
• Categories of exotoxins


  • Cytolytic toxins
  • AB toxins
  • Superantigen toxins

  
• Cytolytic toxins


  Work by degrading cytoplasmic membrane integrity, causing lysis
• AB toxins


  Consist of two subunits, A and B. The B component binds to a host cell surface molecule, facilitating the transfer of the A subunit across the cytoplasmic membrane, where it damages the cell
• Superantigen toxins


  Work by stimulating large numbers of immune cells, resulting in extensive inflammation and tissue damage
• Enterotoxins


  Exotoxins whose activity affects the small intestine, generally causing secretion of fluid into the intestinal lumen resulting in vomiting and diarrhea