Microbe-Human Interactions: Health and Disease

Topic 6: Microbe–Human Interactions: Health and Disease

Introduction

  • Microbiology Fundamentals: A Clinical Approach (2024 Release)
    • Authors: Marjorie Kelly Cowan, Heidi Smith
    • © McGraw Hill LLC. All rights reserved.
    • Learning changes everything.®

Relationships Among Microbes and the Human Host

  • Holobiont: Symbiotic relationship between host and microbiome that maintains a dynamic equilibrium.

Key Vocabulary in Microbiology

  • Normal Biota:

    • Definition: Diverse microbes living on/in the body critical for health.
    • Also known as: Resident/indigenous biota or normal flora.
    • Types of microbes: Bacteria, fungi, protozoa, and viruses.

  • Colonization: Growth of microbes in the body.

  • Infection: When pathogenic microorganisms penetrate host defenses, enter tissues, and multiply.

  • Disease: Pathologic state due to damage or disruption of host tissues and organs (deviation from health).

  • Infectious Disease: Caused by microbes or their products.

  • Endogenous Infections: Resident biota opportunistically enter previously sterile sites.

    • Example: Streptococcus pneumoniae can cause pneumococcal pneumonia in AIDS patients.

Human Microbiome Project (HMP)

  • Global research initiative since 2007 to:
    • Characterize healthy human microbiomes.
    • Identify differences in microbiome composition among diseases.
    • Utilize genomic sequencing to gather extensive data.
    • Human cells contain 21,000 protein-encoding genes.
    • Microbiota consist of millions of protein-encoding genes.
    • Microbes found in locations once considered sterile.
    • Example statistic: 100 million viruses per gram of human feces.
    • Healthy individuals harbor potentially dangerous pathogens in low numbers.
    • Gut microbiota composition influences overall health.

Factors That Weaken Host Defenses

  • Conditions that increase susceptibility to infection:
    • Old age and extreme youth (infancy, prematurity).
    • Genetic and acquired defects in immunity (e.g., AIDS).
    • Surgery and organ transplants.
    • Underlying diseases (cancer, liver malfunction, diabetes).
    • Chemotherapy/immunosuppressive drugs.
    • Physical and mental stress.
    • Pregnancy.
    • Presence of other infections.

Microbial Benefits in Infants

  • Benefits of microbiota in newborns:
    • Influence development of organs and immune system.
    • Microbial antagonism: beneficial microbes combat intruders.
    • Prevention of harmful organism overgrowth.

Resident Biota in Humans

  • Key Anatomical Sites and Common Genera:

    • Skin:

    • Gram-positive: Staphylococcus (S. aureus), Propionibacterium,
      Streptococcus, Corynebacterium, Lactobacillus.

    • Gram-negative: Bacteroides, Prevotella, Haemophilus.

    • Fungi: Candida.

    • Biota varies by skin location and age; ~4% carry S. aureus.

    • GI Tract:

    • Oral Cavity:

      • Gram-positive: Streptococcus predominates; Actinomyces, Corynebacterium.
      • Gram-negative: Haemophilus, Prevotella, Veillonella, Bacteroides, Moraxella.
      • Protozoa: Entamoeba.
      • Result: enormous microbial colonization in saliva.

    • Intestinal Tract:

      • Dominant Gram-negative: Bacteroides, Prevotella.
      • Less Gram-positive: Streptococcus, Lactobacillus.
      • Fungal: Candida.
      • E. coli is prevalent but in low abundance.

    • Respiratory Tract:

    • Nose: Primarily Gram-positive bacteria like Propionibacterium and Staphylococcus.

    • Throat: Similar to oral cavity contents.

    • Lungs: Previously thought sterile.

    • Vagina:

    • Gram-positive: Lactobacillus predominates in reproductive years; changes with hormonal variations.

    • Urinary Tract:

    • Gram-positive: Lactobacillus predominant.

Pathogenicity and Virulence

  • Virulence: Degree of pathogenicity or severity of disease caused by a microbe.
  • Pathogen: Microbe that can cause disease.
  • Distinction between true pathogens (cause disease in healthy individuals) and opportunistic pathogens (cause disease when host is compromised).
  • Virulence Factors: Structures/abilities of pathogens that enable disease causation (invasion, toxin production, allergenicity).
  • Infectious Dose (ID): Minimum number of microbes for infection to occur; smaller ID correlates with greater virulence.

Host-Related Variables Affecting Infection Outcomes

  • Three primary host-related variables:
    • Genetics: Variations affect host responses to infection.
    • Microbial exposure: Varies among individuals.
    • Overall health status: Affects susceptibility and disease progression.

Polymicrobial Infections

  • Recognition that many infections are polymicrobial (involving multiple microbes).
  • Example: Influenza (viral) followed by pneumonia (bacterial).

Biosafety Levels as per CDC

  • Biosafety Levels established based on the risk posed by pathogens:
    • Level 1: Low-risk pathogens; standard practices sufficient.
    • Examples: Micrococcus luteus, Bacillus megaterium.
    • Level 2: Moderate-pathogens; specific training and equipment required.
    • Examples: Staphylococcus aureus, E. coli.
    • Level 3: Severe pathogens; containment practices required.
    • Examples: Mycobacterium tuberculosis.
    • Level 4: Hazardous microbes; maximum containment.
    • Examples: Ebola virus, Marburg virus.

Pathogenesis of Microbial Disease

Steps for Microbial Infestation:

  1. Finding a Portal of Entry
    • Portals: Skin, GI tract, respiratory tract, urogenital tract.
    • Sources: Exogenous (from environment) and endogenous (from normal biota).
  2. Attaching Firmly
    • Adhesion: Process involving adhesins (structures that bind to host receptors).
    • Quorum Sensing: Microbial communication that aids infection establishment.
  3. Surviving Host Defenses
    • Strategies include avoiding phagocytosis, surviving inside phagocytes.
  4. Causing Damage/Disease
    • Mechanisms of damage: Direct (toxins or enzymes), indirect (host response), and epigenetic changes.
  5. Exiting the Host
    • Portals include respiratory tract, skin, fecal matter, urogenital tract, and blood.

Damage Mechanisms by Microbes

  • Types of damage include:
    • Enzymatic breakdown of host defenses (ex: mucinase, hyaluronidase).
    • Eliciting excessive immune response leading to collateral damage.
    • Inducing epigenetic modifications affecting host cell functions.

Infection Patterns Overview

  • Localized Infection: Remains near the portal of entry (e.g., boil).
  • Focal Infection: Spreads to distant tissues (e.g., tuberculosis).
  • Systemic Infection: Affects multiple systems (e.g., anthrax).
  • Mixed Infection: Presence of multiple microbes at the same site (e.g., gas gangrene).

Signs and Symptoms of Infection

  • Syndrome: Combination of signs (objective observation) and symptoms (subjective experience) indicative of disease.
  • Ex: Leukocytosis: Increased white blood cell count due to infection; Leukopenia: Decreased count.

Course of Infection and Disease

  • Incubation period: Time from exposure to symptom onset; varies widely.
  • Prodromal stage: Mild symptoms appear; general malaise.
  • Acute phase: Peak infection activity; severe symptoms.
  • Convalescent stage: Recovery begins; symptoms decline.
  • Continuation phase: Organism persists or symptoms continue after pathogen is cleared.

Surveying and Reporting Diseases

  • Epidemiology studies disease patterns, focusing on frequency, distribution, and determinants.
  • Notifiable Diseases: Must be reported to health authorities; Examples: cholera, rabies, measles.

Tools for Understanding Epidemiology

  • Incidence: New cases in a specified time period.
  • Prevalence: Total existing cases.
  • Morbidity: Number of affected individuals.
  • Mortality: Number of deaths due to disease.
  • R0 (Reproductive Rate): Average number of secondary cases per infected individual.
  • Case Fatality Rate (CFR): Proportion of individuals who die from a specific disease.

Herd Immunity

  • Occurs when a large portion of a population becomes immune, thus reducing disease spread.
  • Factors that hinder achieving herd immunity include vaccine hesitancy and the epidemiological potential for mutation in pathogens.

Bioterrorism

  • Definition: The intentional or threatened use of microbes/toxins to cause fear or illness.

Conclusion

Essential Concepts to Review:

  • Definitions and examples of key microbiological terms (symbiosis, host, pathogen, etc.).
  • Pathogenicity, virulence, infectious dose, disease transmission, epidemiological terms.
  • Important figures in the history of microbiology and epidemiology, including their contributions to the field.
  • Review of Koch's Postulates and when they may not apply.