Mechanisms of Pathogenicity: Study Guide for Chapter 15

Mechanisms of Pathogenicity

Terminology

Understanding the mechanisms by which pathogens cause disease requires familiarity with key terms related to pathogenicity and virulence. These terms describe the process of infection, the ability of microorganisms to cause disease, and their destructive potential.

• Portals of Entry: The routes by which pathogens enter the host, such as skin, respiratory tract, gastrointestinal tract, and genitourinary tract.

• Pathogenicity: The ability of a microorganism to cause disease in a host.

• Virulence: The degree of pathogenicity; a measure of the destructive capacity of the pathogen.

• Portals of Exit: The routes by which pathogens leave the host, including respiratory secretions, feces, urine, and blood.

Pathogenic Factors

Pathogens possess various factors that enable them to invade hosts, evade immune responses, and cause disease. These factors include structural components, enzymes, and toxins.

• Capsules: Polysaccharide layers surrounding some bacteria, protecting them from phagocytosis by host immune cells.

• Hemolysins: Enzymes that lyse red blood cells, releasing iron for bacterial growth.

• Leukocidins: Enzymes that destroy white blood cells, weakening host defenses.

• Coagulases: Enzymes that induce clotting of blood, allowing bacteria to evade immune detection.

• Kinases: Enzymes that dissolve blood clots, facilitating the spread of bacteria.

• Hyaluronidase: Enzyme that breaks down hyaluronic acid in connective tissue, aiding in tissue invasion.

• Siderophores: Molecules that bind and transport iron from the host, essential for bacterial metabolism.

• IgA Proteases: Enzymes that destroy IgA antibodies, reducing mucosal immunity.

• M Protein of Streptococcus: Surface protein that aids in attachment and resistance to phagocytosis.

• Fimbriae: Hair-like appendages used for attachment to host cells, e.g., Neisseria gonorrhoeae.

• Cell Walls: Structural features, such as those in Mycobacteria, that resist phagocytosis.

Toxins

Toxins are potent molecules produced by pathogens that directly damage host tissues or disrupt physiological processes. They are classified as exotoxins, endotoxins, and superantigens.

• Exotoxins: Protein toxins secreted by bacteria, usually heat labile and highly toxic. Most are A-B toxins, where the A component is active and the B component facilitates adherence to host cells.

  • Cytotoxins: Kill host cells. Example: Diphtheria toxin (Corynebacterium diphtheriae) inhibits protein synthesis, leading to cell death.

  • Neurotoxins: Affect nerve cells. Example: Botulinum toxin inhibits acetylcholine release, causing paralysis; tetanus toxin blocks relaxation pathway in muscles, causing sustained contraction.

  • Enterotoxins: Target intestinal cells. Example: Cholera toxin (Vibrio cholerae) disrupts membrane function, causing massive loss of electrolytes and water.

• Endotoxins: Lipid A component of lipopolysaccharide (LPS) in Gram-negative bacteria. Heat stable and less toxic than exotoxins, but can cause septic shock when released in large amounts. Shock results from phagocytes releasing tumor necrosis factor (TNF), increasing capillary permeability and leading to blood volume loss and systemic infection.

  • Septic Shock: Characterized by hypotension, organ failure, and high mortality rate (up to 50%).

• Superantigens: Bacterial proteins that non-specifically stimulate T cell proliferation and cytokine release, including TNF. This can result in symptoms similar to endotoxin-induced shock. Produced by specific strains of Streptococcus pyogenes and Staphylococcus aureus.

Examples and Applications

• Example: The capsule of Streptococcus pneumoniae is essential for its virulence, as it prevents phagocytosis.

• Example: Staphylococcus aureus produces coagulase, which forms clots around the bacteria, protecting them from immune cells.

• Example: Cholera toxin causes severe watery diarrhea by disrupting ion transport in intestinal epithelial cells.

Summary Table: Pathogenic Factors and Their Functions

Factor	Function	Example Organism
Capsule	Prevents phagocytosis	Streptococcus pneumoniae
Hemolysin	Lyses red blood cells	Streptococcus pyogenes
Leukocidin	Destroys white blood cells	Staphylococcus aureus
Coagulase	Induces clot formation	Staphylococcus aureus
Kinase	Dissolves clots	Streptococcus pyogenes
Hyaluronidase	Breaks down connective tissue	Streptococcus species
Siderophore	Acquires iron from host	Various bacteria
IgA Protease	Destroys IgA antibodies	Neisseria gonorrhoeae
M Protein	Attachment, resists phagocytosis	Streptococcus pyogenes
Fimbriae	Attachment to host cells	Neisseria gonorrhoeae
Cell Wall	Resists phagocytosis	Mycobacterium tuberculosis
Exotoxin	Damages host cells	Corynebacterium diphtheriae, Clostridium botulinum
Endotoxin	Induces shock	Gram-negative bacteria
Superantigen	Overstimulates immune response	Streptococcus pyogenes, Staphylococcus aureus

Key Equations

• Virulence Measurement: The lethal dose 50 (LD50) is a common quantitative measure of virulence. $LD_{50} = \text{Dose required to kill 50\% of test population}$