PATHOGENICITY OF MICROORGANISMS

I.  Host Parasite Interactions

   A.  Infection

   B.  Infectious disease

   C.  Pathogen

   D.  Pathogenicity

   E.  Virulence

   F.  Reservoirs and vectors

   G.  Mode of transmission

II.  Viral Pathogenicity

   A.  Entry

   B.  Tissue tropism

  C.  Cell injury and illness

   D.  Outcome

III.  Bacterial Pathogenesis

   A.  Introduction

• competition with normal flora
  
• must overcome host defenses
  
• bacteria can express a number of different virulence mechanisms

   B.  Mechanisms of attachment

• receptors
  
• adhesions
  
• two main classes of adhesions

     1.  pili (fimbriae)

     2.  glycolyx, capsule, slime layer

     3.  microbial cellular adhesions

• lipoteichoic acid (LTA)
  
• hemagglutinin

   C.  Virulence mechanisms leading to host damage

• invasion
• toxigenicity
• immunologically mediated

     1.  iron scavenging

        a) Fe in limiting concentrations

        b) siderophores

        c) directly binding iron-containing proteins

        d) lyse RBCs

     2.  factors contributing to spread

• collagenase
  
• hylauronidase
  
• kinase
  
• mucinase
  
• DNase

     3.  toxigenicity

        a) how toxins reach susceptible cells

• following attachment
  
• soluble toxin
  
• intoxication

       b) endotoxin

• LPS

i. structure

 

ii. pathogenicity

        c) exotoxins

i. cytotolytic toxins

   eg. thiol activiated

 

        enzymatic activity

 

ii. toxins with intracellular activity

    eg. increase concentration of cAMP

 

         protein synthesis inhibition

 

 

iii. other or uncharacterized exotoxins

     eg. neurotoxins

 

        4.  immunologically mediated damage

           a) hypersensitivity reactions

• cell mediated immune response
  
• molecular mimicry

 

b) super antigens

• stimulate a large number of T helper cells
  
• e.g. TSST1 and SPE A

 

         5.  pathogenicity islands

• related genes that increase virulence
  
• virulence genes tightly regulated

   D.  Evading the immune system

     1.  Intracellular parasites

        a) obligate or facultative intracellular parasites

        b) mechanisms of survival

           i. escape phagosome


           ii. prevent phagolysosome formation

           iii. tolerant to phagolysosome formation

           iv. prevent oxidative burst

        c) infect adjacent cells

     2.  resist phagocytosis

        a) capsule

        b) M protein

        c) protein A

     3.  kill phagocyte

     4.  immunosuppression

        a) global

        b) nonspecific immune stimulation

     5.  change surface antigen

• antigenic variation

     6.  shedding surface antigen

     7.  coating surface antigen

        a) capsule

        b) miscellaneous

           i. bind Fc portion of antibody

          ii. coat surface with host antigens

     8.  antibody proteases

Last updated June 19, 2007.