Principles of Disease and Epidemiology: Microbial Mechanisms of Pathogenicity (Chapter 15)

Pathogenicity and Virulence

Definitions and Concepts

Pathogenicity refers to a microbe's ability to cause disease by overcoming host defenses. Virulence is the degree of pathogenicity, indicating how severe or effective a microbe is at causing disease. For a microbe to be pathogenic, it must:

• Gain access to the host

• Penetrate or evade host defenses

• Damage host tissues

Understanding these mechanisms is fundamental to microbiology and disease prevention.

How Microbes Enter a Host

Principal Portals of Entry

Pathogens enter the human body through specific portals of entry. The main portals are:

• Mucous membranes

  • Respiratory tract: Most common; includes inhalation of airborne microbes.

  • Gastrointestinal tract: Entry via contaminated food, water, or fingers.

  • Genitourinary tract: Common for sexually transmitted diseases (e.g., HIV, gonorrhea, syphilis).

  • Conjunctiva of the eye.

• Skin: Generally impermeable unless there are openings (hair follicles, sweat glands).

• Parenteral route: Microbes deposited directly under the skin or mucous membranes due to injury (punctures, injections, bites, cuts, wounds, surgery).

Factors Affecting Disease Causation

• Preferred portal of entry: Some pathogens must enter through a specific portal to cause disease.

• Number of invading microbes: The likelihood of disease increases with the number of microbes. Two key measures:

  • $ID_{50}$: Infectious dose for 50% of the test population

  • $LD_{50}$: Lethal dose (of a toxin) for 50% of the test population

• Adherence: Microbes must attach to host tissues to establish infection.

Microbial Adherence Mechanisms

Adhesins and Ligands

Attachment between pathogen and host is mediated by surface molecules called adhesins or ligands that bind to specific receptors on host cells. These are typically glycoproteins or lipoproteins found on:

• Glycocalyx: e.g., Streptococcus mutans

• Fimbriae: e.g., Escherichia coli

Adherence is crucial for biofilm formation, which enhances microbial survival and resistance.

Penetration or Evasion of Host Defenses

Virulence Factors

After attachment, pathogens must overcome host defenses. Key virulence factors include:

• Capsules: Prevent phagocytosis (e.g., Streptococcus pneumoniae, Haemophilus influenzae, Bacillus anthracis).

• Cell Wall Components:

  • M protein: Resists phagocytosis (Streptococcus pyogenes).

  • Opa protein: Inhibits T helper cells (Neisseria gonorrhoeae).

  • Mycolic acid: Waxy lipid resists digestion (Mycobacterium tuberculosis).

• Enzymes:

  • Coagulase: Coagulates fibrinogen

  • Kinases: Digest fibrin clots

  • Hyaluronidase: Hydrolyzes hyaluronic acid

  • Collagenase: Hydrolyzes collagen

  • IgA proteases: Destroy IgA antibodies

• Antigenic Variation: Pathogens alter their surface antigens, rendering antibodies ineffective.

• Host Cell Cytoskeleton Penetration:

  • Invasins: e.g., Salmonella alters host actin to enter cells.

  • Actin-based movement: e.g., Listeria uses actin to move between cells.

How Pathogens Damage Host Cells

Mechanisms of Damage

Once inside, pathogens can damage host cells in several ways:

1. Using the host's nutrients: e.g., Siderophores are proteins secreted by pathogens to bind iron more tightly than host cells, depriving the host of iron needed for hemoglobin synthesis.

2. Direct damage: Pathogens use host nutrients and produce waste products, causing cell and tissue damage.

3. Producing toxins: Toxins are the primary cause of damage in many infections.

4. Inducing hypersensitivity reactions (not covered in detail for this test).

The Production of Toxins

Key Definitions

• Toxin: Substance that contributes to pathogenicity.

• Toxigenicity: Ability to produce a toxin.

• Toxemia: Presence of toxin in the host's blood.

• Toxoid: Inactivated toxin used in a vaccine.

• Antitoxin: Antibodies against a specific toxin.

Types of Toxins

• Exotoxins:

  • Produced and released by live cells; secreted into surrounding media.

  • Proteins; harmful in small amounts due to repeated action.

  • Produced by both Gram-positive and Gram-negative bacteria.

  • Water-soluble; carried in body fluids and blood.

  • Highly specific in their effects.

  • Can be neutralized by antitoxins; toxoids used in vaccines.

• Endotoxins:

  • Part of the outer membrane of Gram-negative bacteria (LPS layer).

  • Released when cells die; cause symptoms by stimulating macrophages to release cytokines.

  • Not proteins; cannot be neutralized by antitoxins or used as toxoid vaccines.

  • Can cause fever, weakness, and endotoxin shock (e.g., Salmonella typhi).

  • Tested using the Limulus amebocyte lysate (LAL) assay: blood of horseshoe crabs contains amebocytes that lyse in the presence of endotoxin, producing a clot.

Types of Exotoxins

• A-B toxins: Contain an enzyme component (A part) and a binding component (B part). Example: Diphtheria toxin.

• Membrane-disrupting toxins: Lyse host cells by disrupting plasma membranes.

  • Leukocidins: Kill phagocytic leukocytes.

  • Hemolysins: Kill erythrocytes by forming protein channels.

  • Streptolysins: Hemolysins produced by streptococci.

• Superantigens: Cause intense immune response due to release of cytokines from host cells (T cells). Symptoms include fever, nausea, vomiting, diarrhea, shock, and death.

• Genotoxins: Damage DNA, causing mutations, disrupting cell division, and potentially leading to cancer.

Portals of Exit

How Pathogens Leave the Host

Pathogens exit the host through specific portals, which are often the same as the portals of entry:

• Respiratory tract: Coughing and sneezing

• Gastrointestinal tract: Feces and saliva

• Genitourinary tract: Urine and vaginal secretions

• Skin

• Blood: Arthropods that bite; needles or syringes

Summary Table: Microbial Mechanisms of Pathogenicity

Portals of Entry	Penetration/Evasion of Host Defenses	Damage to Host Cells	Portals of Exit
Mucous membranes Skin Parenteral route	Capsules Cell wall components Enzymes Antigenic variation Intracellular growth	Siderophores Direct damage Exotoxins/Endotoxins Cytopathic effects	Respiratory tract Gastrointestinal tract Genitourinary tract Skin Blood

Key Equations

• $ID_{50}$: Infectious dose for 50% of the test population

• $LD_{50}$: Lethal dose (of a toxin) for 50% of the test population

Additional info:

• Biofilm formation is a major factor in microbial persistence and resistance to host defenses and antibiotics.

• Endotoxin shock is a severe, life-threatening condition caused by the release of large amounts of endotoxins, leading to systemic inflammation and organ failure.