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