Mechanisms of Pathogenicity

Mechanisms of Pathogenicity

  • Chapter Overview

    • Topics: Microbiome, Host-Microbe Interactions, Pathogenicity, Opportunistic Infections, Virulence Factors, Toxins, Quorum Sensing, Stages of Infectious Disease.

Microbiota

  • Definition

    • Microbiota refers to microorganisms that inhabit our bodies, replacing the outdated term "microflora."

  • Types

    • Normal Microbiota: Also known as resident microbiota; permanent residents in specific body areas.

    • Transient Microbiota: Temporary inhabitants; can be present under certain conditions but not permanently.

  • Acquisition of Microbiota

    • All sterile before birth, acquiring microbiota from the birthing process and environment afterward.

    • Differences in microbiota based on the method of delivery (vaginal vs. cesarean).

Host-Microbe Interactions

  • Contamination: Presence of an organism without causing harm.

  • Infection: Microorganism multiplies in host, potentially leading to illness (e.g., staph aureus on a cut).

  • Disease: Health disturbance caused by infection; symptoms worsen and disrupt normal bodily functions.

  • Infestation: Term reserved for parasitic infections (e.g., tapeworms).

Pathogenicity

  • Definition: Capability of an organism to cause infection or disease.

  • Factors Affecting Pathogenicity:

    • Ability to invade and multiply in hosts.

    • Quantity of microbes needed for infection (e.g., Shigella can cause illness from one cell).

    • Ability to evade the immune system (e.g., HIV infects immune cells).

Opportunistic Infections

  • Definition: They take advantage of a situation…when they see an opportunity they take it.

  • Factors for Opportunistic Infections:

    • Immunocompromised individuals: Vulnerabilities due to age, disease, or medications.

    • Unusual locations: Normal microbiota can cause infections if introduced into wrong sites (e.g., E. coli in the urinary tract).

    • Disturbance of Normal Microbiota: Antibiotics can wipe out beneficial microbiota, allowing opportunistic pathogens to flourish (e.g., yeast infections after antibiotics for UTIs).

Types of Infectious Diseases

  • Communicable Diseases: Spread easily from person to person.

    • Contagious: Highly communicable (e.g., influenza).

    • Less contagious examples (e.g., pneumonia).

  • Non-communicable Diseases: Cannot be spread person to person (e.g., tetanus, malaria).

Virulence Factors

  • Definition: Factors that help pathogens cause disease.

  • Examples:

    • Pili: Used for attachment to host tissues (e.g., Neisseria gonorrhoeae).

    • Glycocalyx: Enables adherence to surfaces and evasion of the immune system (slime layer vs. capsule).

    • Enzymes:

    • Urease: Neutralizes stomach acid, reaches the epithelial cells to create infections.

    • Hyaluronidase: Breaks down connective tissue, facilitating spread.

    • Collagenase: Destroys collagen, aiding penetration into tissues.

Toxins

  • Endotoxins: Found in gram-negative bacteria, released upon death (e.g., E. coli). Associated with low toxicity but serious effects.

    • Causes endotoxin shock

  • Exotoxins: Secreted by living bacteria, highly toxic (e.g., botulinum toxin), found more with gram positive bacteria but can be found in gram negative.

    • Categories:

    • Neurotoxins: Affect the nervous system (e.g., clostridium botulinum and clostridium tetani).

    • Cytotoxins: Affect cells (e.g., hemolysins- staphylococcus, streptococcus. leukocidins- staphylococcus, streptococcus).

    • Enterotoxins: Affect intestinal cells (e.g., vibrio cholera + staphylococcus aureus).

  • Mycotoxins: Fungal toxins causing health issues (e.g., aflatoxins linked to liver cancer).

    • aflatoxin: produced by aspergillus flavus; grows on grains and nuts, ha to be regulated,

    • ergot alkaloids: produced by a different mold, grows on grains and rye

Quorum Sensing

  • Definition: Communication among bacteria using chemical signaling molecules.

  • Importance in coordinating group behavior and developing strategies against hosts.

  • Concept of anti-quorum sensing molecules as potential broad-spectrum antibiotics.

Stages of Infectious Disease

  1. Incubation Period: Time from infection to symptom onset; number of pathogens is low but increasing.

  2. Prodromal Phase: Mild signs/symptoms; contagious.

  3. Period of Illness: Most acute symptoms; pathogen loads peak.

  4. Period of Decline: Symptoms decrease; susceptible to secondary infections.

  5. Period of Convalescence: Recovery phase; pathogen levels drop back to normal.

Additional Concepts

  • Antigenic Drift: Minor antigenic changes over time due to mutations.

  • Antigenic Shift: Major changes resulting from gene mixing between different strains, leading to new viral strains (e.g., influenza).