Lecture on Pathogenicity and Microbial Invasion

Definition of Key Terms

• Pathogenicity: The ability of a microbe to cause disease.
• Virulence: The degree of pathogenicity or how severe the pathogenic effect is.

Portals of Entry

• Mucous Membranes: Includes inner linings like the mouth and respiratory tract.
• Skin: Protects internal body, but breaks like cuts can allow microbe entry.
• Parental Route: Direct entry into tissues, bypassing protective barriers.
• Preferred Portal of Entry: Specific pathogens have preferred entry routes, e.g., skin or respiratory tract.

Infectious and Lethal Doses

• ID50: Infectious dose for 50% of a population.
• LD50: Lethal dose for 50% of a population.
• Example: Bacillus anthracis varies in its infectious dose based on entry route.

Adherence and Invasion

• Adhesins: Surface molecules on pathogens that attach to host cells.
• Biofilms: Microbial communities that protect and adhere via EPS (extracellular polymeric substance).

Mechanisms to Evade Immune System

• Capsules: Surround bacterial cell wall, impairing phagocytosis.
  • Examples: Streptococcus pneumoniae, Haemophilus influenzae, Bacillus anthracis.
• M Proteins, Opa Proteins, Mycolic Acid: Aid in attachment and resist phagocytosis.

Enzyme-Based Defense Mechanisms

• Coagulases: Induce blood clotting.
• Kinases: Prevent blood clotting.
• Hyaluronidase, Collagenase: Break down host tissues.
• IgA Proteases: Degrade antibodies.

Antigenic Variation

• Pathogens can alter surface proteins to evade immune detection.

Invasion of Host Cells

• Invasins: Rearrange cytoskeleton (actin) to promote entry.
• Survival in Phagocytes: Some bacteria can survive phagocytosis and evade destruction.

Damaging Host Cells

• Siderophores: Bind and sequester iron from host.
• Toxins:
  • Exotoxins: Protein toxins causing diverse effects (e.g., AB toxins, membrane-disrupting toxins, superantigens).
    • AB Toxins have A (active) and B (binding) components.
  • Endotoxins: Lipopolysaccharide (LPS) from gram-negative bacteria, particularly toxic lipid A component.

Viral Pathogenic Effects

• Cytopathic Effects: Include inhibition of host cell functions, inclusion body formation, and syncytium formation.
• Interferons: Signal uninfected cells to mount defenses.

Fungi, Protozoa, and Helminths

• Aflatoxins: Carcinogenic toxins from fungi.
• Mycotoxins: Neurotoxins from mushrooms.
• Waste Products: Protozoa produce waste leading to symptoms.
• Antigenic Variation: Common in protozoa and helminths.

Portals of Exit

• Respiratory Tract: Via coughing and sneezing.
• Gastrointestinal Tract: Through feces and saliva.
• Genitourinary Tract: Via urine and sexual secretions.
• Skin and Blood: Through wounds or insect bites.

Summary

• Microbes enter, adhere, invade, evade defenses, damage host cells, and exit, often through the same portal as entry.