14. Microbial Mechanisms to Pathogenicity

• Pathogenicity


  The ability to cause disease by overcoming host defenses
• Virulence


  Degree of pathogenicity
• Portals of entry for pathogens

  • Mucous membranes
  • Skin
  • Parenteral route
• How microorganisms enter a host

  1. Gain access to host

  2. Adhere to host tissues

  3. Penetrate or evade host defenses

  4. Damage host tissue
• Some diseases are caused by the accumulation of microbial waste products (dental caries and acne)
• Mucous membranes


  Lining the respiratory, gastrointestinal, genitourinary tracts, and conjunctiva
• Skin


  Largest organ of the body, important defense against disease, microbes can gain access through openings
• Parenteral route


  When pathogens are deposited directly into the tissues beneath the skin or into mucous membranes when these barriers are penetrated or injured
• Diseases transmitted through parenteral route

  • HIV
  • Hepatitis viruses
  • Tetanus
  • Gangrene
• Preferred portal of entry

  Many pathogens have a prerequisite portal of entry to cause disease
• Pathogens with preferred portals of entry

  • Salmonella Typhi (swallowed)
  • Streptococcus pneumonia (inhaled)
  • Yersinia pestis
  • Bacillus anthracis
• LD50


  Lethal dose for 50% of the inoculated hosts
• ID50


  Infectious dose for 50% of the inoculated hosts
• Adherence


  Necessary step in pathogenicity, attachment of pathogens to host tissues at their portal of entry
• Adhesins


  Surface molecules on pathogens that bind to complementary receptors on host cells
• Mannose is the most common receptor for adhesins
• Biofilms


  Provide attachment and resistance to antimicrobial agents

  
• Capsules


  • Prevent pathogens from being phagocytized
  • Examples: Streptococcus pneumoniae, Klebsiella pneumoniae, Haemophilus influenzae, Bacillus anthracis
• Cell wall components


  • Facilitate adherence or prevent phagocytosis
  • Examples: Streptococcus pyogenes M protein, Neisseria gonorrhoeae fimbriae and Opa protein, Mycobacterium tuberculosis waxy cell wall
• Enzymes


  • Protect local infections in fibrin clots
  • Spread infections by destroying blood clots, connective tissue, and IgA antibodies
• Antigenic variation


  • Avoid host antibodies by changing antigen expression
  • Examples: Neisseria gonorrhoeae, Influenzavirus, Trypanosoma brucei gambiense
• Penetration into host


  • Bacteria alter host cell cytoskeleton to enter
  • Examples: Salmonella, E. coli, Shigella, Listeria
• Biofilms


  • Protect bacteria from phagocytosis and immune recognition

  
• Siderophores


  Proteins secreted by pathogens to obtain iron from host
• Direct damage


  Host cells destroyed by pathogen metabolism and multiplication
• Exotoxins


  Poisonous substances produced by bacteria and released into the surrounding medium
• 1. B toxins


  Consist of an active component that inhibits a cellular process and a binding component that attaches to the target cell
• Membrane-disrupting toxins


  Cause cell lysis
• Superantigens


  Cause release of cytokines, leading to fever, nausea, and other symptoms
• Endotoxins


  Lipid A component of gram-negative bacterial cell walls, released on cell death and causing fever and shock
• Plasmids


  May carry genes for virulence factors like antibiotic resistance, toxins, capsules, and fimbriae
• Lysogenic conversion


  Can result in bacteria with virulence factors like toxins or capsules
• Viruses


  • Avoid immune response by growing inside cells
  • Gain access via host cell receptors
  • Cause cytopathic effects like cell death, mitosis inhibition, inclusion bodies, cell fusion, antigenic changes, chromosomal changes, and transformation
• Fungi, protozoa, helminths, algae


  • Cause symptoms through capsules, toxins, allergic responses, damage to host tissue, metabolic waste products, and changing surface antigens

  
• Portals of exit for pathogens

  • Respiratory tract
  • Gastrointestinal tract
  • Genitourinary tract
  • Skin or wound
  • Infected blood
• Diseases transmitted through portals of exit

  • Yaws
  • Impetigo
  • Ringworm
  • Simplexvirus
  • Warts
  • Yellow fever
  • Plague
  • Tularemia
  • Malaria
  • AIDS
  • Hepatitis B