pathogenicity of microorganisms

• Pathogenic Microorganism:

  • Capable of causing disease
  • Some microorganisms are unequivocally pathogenic, while others are generally harmless
• 1. Reactivation of a latent infection
  2. The second stage of an infection

  (Ex: a) Mycobacterium tuberculosis - tuberculosis

  1. Treponema pallidum - syphilis)

  • Mixed: Two or more bacteria infecting the same tissue (Ex: Pelvic inflammatory disease may be initiated by infection with N. gonorrhoeae or C. trachomatis but other organisms including anaerobes play important roles in progression of the disease)
  • Pyogenic: Pus forming (Ex: staphylococcal and streptococcal infections)
• Terminologies:

  • Subclinical (Asymptomatic): An infection with no detectable symptoms (Ex: asymptomatic gonorrhea)
  • Latent: An infection with the potential to become active at some time (Ex: Treponema pallidum - syphilis, Mycobacterium tuberculosis - tuberculosis)
  • Opportunistic: An infection due to an organism that generally does not cause disease unless normal host defenses are compromised (Ex: Pneumocystis pneumonia in patients with HIV)
  • Primary: Infection by an organism that may become latent and later cause other disease manifestations (Ex: Treponema pallidum - syphilis)
  • Secondary:
• • Fulminant: Infections that occur suddenly and intensely (Ex: Necrotizing fasciitis from Streptococcus pyogenes, also called "flesh-eating bacteria")
• Reservoir:

  • Source of the microorganism

  1. Humans

  2. Animals, Insects, Birds, Fish, Reptiles, Plants

  3. Environment (non-living objects)
• Transmission:

  • Mode or means how a microorganism can be acquired

  1. Human to human - Direct

  2. Animal, insect (vectors) / carriers - to - human

  3. Environment (non-living object)
• Pathogenicity:

  • Interaction between microorganism and human/host
• Virulence Factors:

  • Characteristics of a bacterium that enhance its pathogenicity
  • Properties that enable a microorganism to establish itself and replicate on or within a specific host
• Steps in the Infectious Process:

  1. Entry into the host, with evasion of host primary defenses

  • Via the respi, GI, or GUT or through skin that has been cut, punctured, or burned

  2. Adherence of the microorganism to host cells

  • Pili / Fimbriae
  • Surface adhesion molecules
  • Evasion of the host's immune system by changing surface antigens

  3. Invasion of the host

  • Enter host cells or penetrate mucosal surfaces
  • Facilitated by bacterial enzymes (collagenase and hyaluronidase) that degrade components of the extracellular matrix

  4. Propagation of the organism
• • Iron sequestering through iron-binding compounds (siderophores)
  • Incorporation of iron into essential compounds such as cytochromes

  5. Damage to host cell by bacterial toxins or immune response of the host

  • Virulence factors that inhibit phagocytosis (capsule external to the cell wall)
  • Bacterial toxins:

  > Exotoxins - proteins secreted by G+ & G- bacteria

  > Endotoxins - integral cell wall component of G- bacteria (LPS)

  6. Progression or Resolution of the disease
• Host-mediated Pathogenesis:

  • The pathogenesis of many bacterial infections is caused by the host response rather than by bacterial factors
  • Tissue damage in these infections is caused by various cytokines released from lymphocytes, macrophages, and polymorphonuclear leukocytes
  • Intense host response can lead to tissue destruction, allowing remaining bacteria to proliferate