Microbial Mechanisms of Pathogenicity: How Pathogens Cause Disease

Microbial Mechanisms of Pathogenicity

Introduction to Pathogenicity and Virulence

Pathogenicity is the ability of a microorganism to cause disease by overcoming host defenses. Virulence refers to the degree of pathogenicity, indicating how severe the disease caused by the microbe can be. For a microbe to cause disease, it must:

• Gain access to the host

• Penetrate or evade host defenses

• Damage host tissues

Understanding these mechanisms is fundamental to microbiology and infectious disease control.

Portals of Entry

Principal Portals of Entry

Pathogens enter the human body through specific portals of entry:

• Mucous membranes: Includes the respiratory tract (most common), gastrointestinal tract, genitourinary tract, and conjunctiva of the eye.

• Skin: Usually through openings such as hair follicles or sweat glands, as unbroken skin is generally impermeable.

• Parenteral route: Direct deposition beneath the skin or mucous membranes due to injury, punctures, bites, cuts, wounds, or surgery.

Not all microbes that enter the body cause disease; successful infection depends on several factors.

Factors Affecting Pathogenicity

• Preferred portal of entry: Some pathogens can only cause disease if they enter through a specific portal.

• Number of invading microbes: The infectious dose required to cause disease is measured as ID50 (infectious dose for 50% of the population) and LD50 (lethal dose for 50% of the population).

• Adherence: Pathogens must attach to host tissues using surface molecules called adhesins or ligands, which bind to specific receptors on host cells. Examples include glycocalyx (e.g., Streptococcus mutans) and fimbriae (e.g., Escherichia coli).

Penetration or Evasion of Host Defenses

Mechanisms Used by Pathogens

• 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 to evade immune detection.

• Penetration into host cell cytoskeleton: Invasins (e.g., Salmonella) alter host actin to enter cells; Listeria uses actin to move between cells.

How Pathogens Damage Host Cells

Mechanisms of Host Cell Damage

• Using the host's nutrients: Pathogens secrete siderophores to bind iron more tightly than host cells, depriving the host of this essential nutrient.

• Direct damage: Pathogens can damage host cells by using nutrients and producing waste products.

• Production of toxins: The majority of host cell damage is due to toxins.

Definitions Related to Toxins

• 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: Proteins secreted by live bacteria (both Gram-positive and Gram-negative). They are highly specific, water-soluble, and can be neutralized by antitoxins. Types include:

  • A-B toxins: Contain an enzyme (A) and a binding (B) component (e.g., diphtheria toxin).

  • Membrane-disrupting toxins: Lyse host cells by disrupting plasma membranes (e.g., leukocidins, hemolysins, streptolysins).

  • Superantigens: Cause intense immune responses by stimulating cytokine release, leading to fever, shock, and death.

  • Genotoxins: Damage DNA, causing mutations and potentially cancer.

• Endotoxins: Lipopolysaccharides (LPS) found in the outer membrane of Gram-negative bacteria (specifically lipid A). Released only when bacteria die, causing symptoms such as fever, chills, and shock. They are not neutralized by antibodies and do not form effective toxoid vaccines. Example: Salmonella typhi.

Detection of Endotoxins

• Limulus amebocyte lysate (LAL) assay: Uses horseshoe crab blood to detect endotoxins, as amebocytes lyse and form a clot in their presence.

Portals of Exit

How Pathogens Leave the Host

Pathogens exit the host through the same portals as entry, including:

• Respiratory tract (coughing, sneezing)

• Gastrointestinal tract (feces, saliva)

• Genitourinary tract (urine, vaginal secretions)

• Skin

• Blood (via arthropod bites, needles, or syringes)

Summary Table: Microbial Mechanisms of Pathogenicity

Step	Mechanism	Examples
Portals of Entry	Mucous membranes, skin, parenteral route	Respiratory tract: influenza; GI tract: Salmonella; Genitourinary: HIV
Penetration/Evasion	Capsules, cell wall components, enzymes, antigenic variation, invasins	Streptococcus pneumoniae, Neisseria gonorrhoeae, Salmonella
Damage to Host Cells	Siderophores, direct damage, toxins (exotoxins, endotoxins)	Diphtheria toxin, LPS from Gram-negative bacteria
Portals of Exit	Respiratory, gastrointestinal, genitourinary tracts, skin, blood	Coughing, sneezing, feces, urine, blood exposure

Additional info: The included image visually summarizes the sequence of microbial pathogenicity: entry, evasion/penetration, host cell damage, and exit, with representative micrographs of pathogens. This reinforces the stepwise process described in the notes.