Study Notes on Host-Microbe Interactions and Pathogenesis

BSCI 20021: Basic Microbiology

HOST-MICROBE INTERACTIONS AND PATHOGENESIS

CHAPTER 10
Introduction
  • Basic Microbiology principles covered in the context of host-microbe interactions and pathogen existence.
  • Importance of studying these interactions for understanding infections and diseases.
Key Concepts
Infectious Dose (ID₅₀)
  • Definition: The infectious dose (ID₅₀) is the number of microorganisms required to cause disease in 50% of the exposed population.
  • Significance: Indicates the virulence of a pathogen; lower ID₅₀ means higher virulence.
Lethal Dose (LD₅₀)
  • Definition: The lethal dose (LD₅₀) refers to the amount of toxin required to kill 50% of the hosts exposed to it.
  • Implications: Affects the treatment and understanding of toxin potency in various pathogens.
Types of Toxins
Exotoxins
  • Definition: Exotoxins are toxic substances secreted by pathogens into their environment.
  • Characteristics: Highly toxic and can cause significant damage even in small quantities.
Endotoxins
  • Definition: Endotoxins are components of the cell structures of Gram-negative bacteria that are released upon cell death.
  • Impact: Can lead to systemic inflammatory responses, often contributing to septic shock if not managed promptly.
Immune Response to Endotoxins
  • Endotoxins can trigger severe body-wide inflammatory responses, including:
    • Fever
    • Chills
    • Hypotension (low blood pressure)
    • Organ damage
  • Sepsis: A life-threatening organ dysfunction caused by a dysregulated host response to infection, potentially stemming from endotoxin exposure.
Effect of Pathogen Interaction
  • Pathogens do not always enter host cells directly. Instead, they can:
    • Bind to receptors on host cells, altering cell physiology without entering the cell.
    • Hyperactivate B cells or T cells, disrupting normal immune function.
Pathogen Examples and Mechanisms
  • Specific bacteria like enterotoxigenic strains can target the intestines, leading to rehydration issues (e.g., through exposure to enterotoxins).
Superantigens
  • A type of antigen that causes an excessive activation of T cells and can lead to overwhelming immune responses.
  • Mechanism: Binds to MHC and T cell receptor simultaneously, stimulating massive T cell activation and cytokine release, causing damage to host tissues.
Mechanisms of Cell Death by Toxins
  • Toxins can:
    • Form pores in host cell membranes, leading to cell lysis and death.
    • Trigger inflammatory reactions that can cause tissue damage and necrosis.
Bacterial Adherence Mechanisms
Adhesion Structures
  • Bacteria possess various structures for adhesion to host cells, critical for establishing infections. These include:
    • Fimbriae: Hair-like structures facilitating attachment through adhesins, which are specific proteins that bind to host cell receptors.
    • Pili: Similar to fimbriae but are involved in conjugation and sharing genetic material as well as adherence.
  • Importance of adhesion structures:
    • Evasion of host immune responses.
    • Nutrient acquisition from host tissues.
Fimbriae Detailed
  • Fimbriae are adhesion structures made of protein, thin and hair-like, often numerous (dozens or hundreds per cell).
  • Function: Allow specific adhesion to host tissues. Example: Strains causing respiratory infections attach to lung proteins.
Pathogen Mechanisms of Immune Evasion
  • Pathogens use various strategies to evade the immune system, including reduction of detection through:
    • Capsule formation
    • Secreted enzymes (e.g., kinases, coagulases)
    • Diverse toxins that disrupt normal immune functioning.
Reading Assignment
  • Assigned Pages: 315-323
  • Key Focus Areas:
    • Review knowledge on adhesion structures and how bacteria adhere to cells and tissues.
    • Investigate how pathogens enter host cells.
    • Learn about mechanisms of immune system evasion.