Microbiology: Principles of Disease and Epidemiology

Key Topics in Microbiology

  • Human Microbiome
  • Etiology of Infectious Diseases and Koch’s Postulates
  • Classification of Diseases
  • Patterns of Disease
  • Predisposing Factors
  • Development of Disease
  • Spread of Infections
  • Reservoirs of Infection
  • Chain of Transmission
  • Healthcare-Associated Infections
  • Emerging Infectious Diseases
  • Breaking the Chain of Transmission

Terminology

  • Etiology: Cause of a disease
  • Infection: Colonization of the body by pathogens
  • Pathogenesis: Development of disease
  • Disease: Abnormal state where the body is not functioning normally
  • Occurs when pathogens overcome host defenses

Relationship between Microbes and Humans

  • Symbiosis: Living together of different organisms
    • Three types of symbiosis: mutualism, commensalism, parasitism

Human Microbiome and the Host

  • Human body contains 30 trillion cells and over 40 trillion microbes
  • Normal Microbiota:
    • Reside permanently, typically non-pathogenic
    • Established in utero; some evidence of placental microbiome
  • Transient Microbiota: Temporary presence in the body
  • Probiotics: Live microbes ingested for health benefits
  • Prebiotics: Food for probiotics
Benefits of Normal Microbiome
  1. Protects host through microbial antagonism by competing with pathogens
  2. Synthesizes essential nutrients like B vitamins and vitamin K
  3. Stimulates immune response
Consequences of Dysbiosis
  • When microbiome is disrupted, pathogens may thrive
  • Example: Antibiotic treatment can cause imbalance (dysbiosis), leading to infections like C. difficile colitis

Koch's Postulates

  • Developed by Robert Koch to determine the cause of infectious diseases:
    1. Same pathogen in every case of disease
    2. Pathogen isolated and grown in pure culture
    3. Pathogen causes disease when introduced to a healthy host
    4. Pathogen re-isolated from the experimentally infected host

Limitations to Koch’s Postulates

  • Some pathogens cannot be cultured in laboratories
  • Some cause similar signs/symptoms or multiple diseases
  • Ethical considerations in human infections

Classifying Infectious Diseases

Categories
  • Communicable Diseases: Spread from host to host
    • Contagious Diseases: Easily spread (e.g., influenza)
  • Noncommunicable Diseases: Not spread between hosts
Occurrence
  1. Sporadic: Occasional cases
  2. Endemic: Constant presence in a population
  3. Epidemic: Rapid increase in cases in a specific area
  4. Pandemic: Global epidemic (e.g., AIDS, COVID-19)
Severity
  • Acute: Rapid onset, short duration
  • Chronic: Slower onset, longer duration
  • Latent: Periods of inactivity with potential reactivation

Development of Disease

  1. Source of pathogens
  2. Transmission to susceptible host
  3. Invasion: pathogen enters and multiplies
  4. Pathogenesis: pathogen injures the host

Spread of Infections

Components Necessary
  • Reservoir of infection
  • Susceptible host
  • Route of transmission
Reservoirs of Infection
  • Living: Humans (carriers), Animals (zoonoses)
  • Nonliving: Soil, Water, Food

Modes of Disease Transmission

  1. Contact: Direct, indirect, droplet
  2. Vehicle: Airborne, waterborne, foodborne
  3. Vector: Arthropods (mechanical or biological transmission)

Healthcare-Associated Infections (HAIs)

  • Infections acquired during healthcare treatment
  • Affect 1 in 25 hospital patients; contribute to about 70,000 deaths annually
Factors Contributing to HAIs
  • Compromised hosts, drug-resistant pathogens, chain of transmission

Control Measures to Reduce HAIs

  • Standard precautions, Transmission-based precautions, Aseptic techniques
  • Hand hygiene, proper use of PPE, patient education

Emerging Infectious Diseases

  • New or re-emerging diseases with potential increase in incidence
  • Examples: West Nile virus, Methicillin-resistant Staphylococcus aureus (MRSA), Ebola

Epidemiology

  • Study of disease occurrence and transmission in populations
    • Objective: Understand disease etiology, spread, control methods
Nationally Notifiable Diseases
  • Specific diseases that must be reported (e.g., HIV, TB, Zika)

Breaking the Chain of Transmission

  • Remove reservoirs and methods of transmission
    • Examples: Water purification, vector control, safe food handling, hygiene practices