Requirements for Infection

Requirements for a Successful Infection

  • A successful microbial infection involves five key steps:
    • Entry: The pathogen must gain access to the host.
    • Establishment: The pathogen must remain in the host.
    • Defeat Host Defenses: The pathogen must evade or overcome the host's immune responses.
    • Damage the Host: The pathogen causes harm to the host.
    • Exit the Host: The pathogen must be transmitted to a new host.

Portals of Entry

  • A portal of entry is any point at which a pathogen enters the host.
  • Common portals of entry include:
    • Mucous Membranes: Respiratory, gastrointestinal, and genitourinary tracts.
    • Skin: Via breaks or punctures.
    • Parenteral Route: Direct entry through breaks in the skin or mucous membranes.

Mucous Membranes

  • Mucous membranes are in direct contact with the external environment, allowing pathogens to cross and access the body's interior.
  • They are found in the respiratory, gastrointestinal, and genitourinary tracts.

Respiratory Tract

  • The respiratory tract is the number one portal of entry.
  • Potential pathogens include:
    • Staphylococcus
    • Streptococcus
    • Haemophilus
    • Veillonella
    • Candida
  • Most common respiratory pathogens:
    • Adenoviruses
    • Streptococcus pneumoniae
    • Staphylococcus aureus
    • Mycobacterium tuberculosis
    • Bordetella pertussis
    • Influenza
    • Rubella
    • Varicella-zoster

Gastrointestinal Tract

  • The gastrointestinal tract is the second most common portal of entry.
  • Most common gastrointestinal pathogens:
    • Escherichia coli
    • Helicobacter pylori
    • Clostridium botulinum
    • Candida
    • Enterococcus faecalis
    • Poliovirus
    • Shigella spp
    • Rotavirus
    • Salmonella
    • Norovirus

Genitourinary Tract

  • The genitourinary tract includes sexually transmitted infections (STIs).
  • Urinary tract infections are more common in women due to a shorter urethra and proximity to the anus.
  • Most common genitourinary pathogens:
    • Mycobacterium smegmatis
    • Chlamydia trachomatis
    • Neisseria gonorrheae
    • Trichomonas vaginalis
    • Herpesviruses
    • Human Immunodeficiency Virus (HIV)
    • Escherichia coli
    • Candida

Skin

  • The skin is the largest organ of the body with many microorganisms residing on it (normal flora).
  • It is an impermeable barrier unless broken.

Parenteral Route

  • The parenteral route refers to breaks in the skin that allow passage of microbes.
  • Normal flora can cause infection, acting as opportunistic pathogens.
  • Entry depends on cuts, abrasions, injections, wounds, surgical incisions, or insect bites.

Establishment

  • After entering the body, pathogens must find a way to stay in.
  • Pathogens use virulence structures such as capsules or fimbriae to attach to the surface of cells or tissues.
  • Pathogens may also use adhesins to attach to tissue.
    • Plaque on teeth is an example of an adhesion beginning a process resulting in an accumulation of bacteria, called a biofilm.
    • The life activities of the bacteria plus their presence results in the formation of plaque.
  • Spirochetes corkscrew into tissues.

Increasing the Numbers

  • Some pathogens can double their numbers every 20 minutes.
  • An organism that doubles every 20 minutes can reach a massive population in just 24 hours: 2^{72} = 4.72 \times 10^{21}
  • Bacteria reproduce by binary fission, where one cell splits into two, then four, then eight, and so on.

Defeating the Host Defenses

  • Pathogens must avoid, evade, or compromise host defenses to survive.
  • There are two types of defense:
    • Passive defense: Using intrinsic features of the pathogen.
    • Active defense: Attacking the host defenses.

Passive Defense

  • Passive defense is the inherent ability to protect themselves via intrinsic factors.
    • Examples: cell walls, lipopolysaccharide membranes, capsules, M protein.

Cell Walls

  • Gram-positive cell walls:
    • Very thick.
    • M proteins (Streptococcus spp.): Increase adhesion to host tissue and inhibit phagocytosis.
  • Gram-negative cell walls:
    • Very thin.
    • Compensate by having an outer lipopolysaccharide membrane (endotoxin).
      • Endotoxin: Lipid A, which elicits a very powerful immune response.
  • Acid-Fast Organisms:
    • Mycolic Acid (Mycobacterium spp. - tuberculosis, leprae)
      • Inhibits phagocytosis and antibiotics

Capsules

  • Capsules protect against phagocytosis.

Active Defense

  • Active defenses are produced by microbes solely for