Mechanisms of Pathogenicity: How Microorganisms Cause Disease

Mechanisms of Pathogenicity

Introduction

Pathogenicity refers to the ability of a microorganism to cause disease, while virulence describes the degree of pathogenicity. Understanding the mechanisms by which microbes invade, evade host defenses, and cause damage is essential for microbiology students.

Parameters for Microbial Pathogenicity

• Pathogenicity: The capacity of a microbe to cause disease in a host.

• Virulence: The degree of pathogenicity, often measured by the severity of disease or the microbe's ability to invade host tissues.

• Necessary Parameters:

  • Entry into the host

  • Adherence to host tissues

  • Penetration or evasion of host defenses

  • Damage to host tissues

  • Exit from the host to infect new hosts

Portals of Entry

Microorganisms enter the host through specific portals, each associated with different types of infections.

• Mucous Membranes: Respiratory tract (e.g., influenza virus), gastrointestinal tract (e.g., Salmonella), genitourinary tract (e.g., Neisseria gonorrhoeae).

• Skin: Intact skin is a barrier, but some microbes enter through cuts, abrasions, or hair follicles (e.g., Staphylococcus aureus).

• Parenteral Route: Direct deposition into tissues beneath the skin or mucous membranes, such as via punctures, injections, or bites (e.g., rabies virus, HIV).

ID50 and LD50

• ID50 (Infectious Dose 50): The number of microbes required to cause infection in 50% of a test population. Example: Bacillus anthracis has different ID50 values depending on the portal of entry.

• LD50 (Lethal Dose 50): The amount of toxin (or number of organisms) required to kill 50% of a test population. Example: Botulinum toxin has a very low LD50, indicating high potency.

Microbial Adherence to Host Cells

• Adherence: The process by which microbes attach to host tissues, often a prerequisite for infection.

• Adhesins: Surface molecules on pathogens that bind to specific receptors on host cells. Commonly found on fimbriae, pili, or the microbial cell wall. Example: Neisseria gonorrhoeae uses pili to adhere to urogenital epithelium.

Penetration and Evasion of Host Defenses

• Capsules: Glycocalyx layers that protect bacteria from phagocytosis, increasing virulence. Example: Streptococcus pneumoniae and Bacillus anthracis.

• Cell Wall Components:

  • M protein: Resists phagocytosis (e.g., Streptococcus pyogenes).

  • Opa protein: Aids in attachment and entry into host cells (e.g., Neisseria gonorrhoeae).

  • Mycolic acid: Waxy lipid in cell walls of Mycobacterium tuberculosis, resists digestion by phagocytes.

• Exoenzymes: Enzymes secreted by bacteria to aid invasion and evade defenses:

  • Coagulase: Clots fibrinogen in blood, protecting bacteria from phagocytosis.

  • Kinases (e.g., Streptokinase): Digest fibrin clots, allowing spread.

  • Hyaluronidase: Hydrolyzes hyaluronic acid in connective tissue, aiding spread.

  • Collagenase: Breaks down collagen, facilitating invasion.

  • IgA protease: Destroys IgA antibodies, reducing immune defense.

• Antigenic Variation: Pathogens alter their surface antigens to evade immune detection. Example: Influenza virus changes hemagglutinin and neuraminidase; Trypanosoma brucei changes surface glycoproteins.

• Invasins: Surface proteins that induce host cell cytoskeletal changes (membrane ruffling) to facilitate entry.

How Bacteria Cause Damage to Host Cells

1. Using Host Nutrients: Bacteria secrete siderophores to scavenge iron from host proteins, essential for growth.

2. Direct Damage: Bacteria multiply inside host cells, causing lysis and tissue destruction.

3. Production of Toxins:

   • Toxins: Poisonous substances produced by microbes that damage host tissues.

   • Toxigenicity: Ability to produce toxins.

   • Toxemia: Presence of toxins in the blood.

   • Intoxication: Disease caused by ingestion of preformed toxins.

   • Exotoxins: Proteins secreted by bacteria, often enzymes, that disrupt host cell function. Types include:

     • A-B toxins: Consist of an active (A) component and a binding (B) component. The B component binds to the host cell, and the A component exerts the toxic effect.

     • Membrane-disrupting toxins: Cause cell lysis by disrupting plasma membranes. Examples:

       • Leukocidins: Destroy white blood cells.

       • Hemolysins: Lyse red blood cells (e.g., streptolysins from Streptococcus).

     • Superantigens: Cause intense immune responses (e.g., toxic shock syndrome).

     • Genotoxins: Damage host DNA, potentially leading to cancer.

     • Antitoxin: Antibodies that neutralize toxins.

     • Toxoid: Inactivated toxin used in vaccines.

   • Endotoxins: Lipid A component of lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria. Released upon cell lysis, causing fever and shock. Detected by the Limulus amebocyte lysate (LAL) assay.

4. Inducing Hypersensitivity Reactions: Some microbial products trigger excessive immune responses, damaging host tissues.

Role of Plasmids and Lysogeny in Pathogenicity

• Plasmids: Small, circular DNA molecules that can carry genes for antibiotic resistance or toxin production.

• Lysogeny: Integration of bacteriophage DNA into the bacterial genome, which can confer new pathogenic properties (e.g., toxin genes).

Pathogenic Properties of Viruses

• Host Cell Damage: Viruses replicate inside host cells, causing cell death or dysfunction.

• Cytopathic Effects (CPE):

  • Inclusion bodies: Aggregates of viral particles (e.g., Negri bodies in rabies).

  • Syncytium formation: Fusion of infected cells into multinucleated giant cells.

  • Antigenic changes: Induction of new antigens on host cell surfaces, potentially leading to autoimmunity.

  • Loss of contact inhibition: Uncontrolled cell growth, sometimes leading to cancer.

Pathogenic Effects of Fungi

• Trichothecene toxins: Inhibit protein synthesis; found on wall boards and grains.

• Proteases: Enzymes that degrade host proteins.

• Ergot toxin: Alkaloid with LSD-like effects, produced by Claviceps purpurea.

• Mycotoxins: Toxic compounds (e.g., amanitin from Amanita phalloides, the death cap mushroom).

Pathogenic Effects of Protozoa

• Digesting host cells and tissues: Direct destruction of host cells.

• Growth within phagocytes: Some protozoa survive and multiply inside immune cells.

• Antigenic variation: Changing surface antigens to evade immune detection (e.g., Trypanosoma brucei).

Pathogenic Effects of Helminths

• Use of host tissue for growth: Leads to tissue damage and nutrient depletion.

• Mass formation: Large numbers can block organs or vessels (e.g., elephantiasis from Wuchereria bancrofti).

• Toxic waste products: Released by helminths, causing inflammation and allergic reactions.

Pathogenic Effects of Algae

• Saxitoxin: Neurotoxin produced by dinoflagellates, causes paralytic shellfish poisoning and is associated with red tides.

Portals of Exit

Microorganisms leave the host through specific portals to infect new hosts. The portal of exit is often related to the site of infection.

• Respiratory tract: Coughing and sneezing (e.g., influenza virus, Mycobacterium tuberculosis).

• Gastrointestinal tract: Feces and saliva (e.g., cholera, polio).

• Genitourinary tract: Urine and vaginal secretions (e.g., Neisseria gonorrhoeae).

• Skin and wounds: Direct contact or drainage (e.g., Staphylococcus aureus).

• Blood: Arthropod bites, needles (e.g., HIV, malaria).