Lecture 2: How Bacteria Cause Disease - Pathogenicity

Pathogenicity and Infectious Disease

  • Chapter 10 in the textbook covers pathogenic mechanisms and evasion strategies of microorganisms.

Key Concepts

  • Pathogenicity: How bacteria cause disease.
  • Infectious Disease
  • Pathogen
  • Bacterial Mechanisms of Disease
  • Substances Produced by Bacteria That Cause Disease
  • Cellular Damage Caused by Bacteria
  • Bacterial Evasion of the Body's Defense Mechanisms (briefly; more detail in immunity lectures).

Balance with Normal Flora and Environment

  • We coexist with normal flora (bacteria and other organisms that live in/on us).
  • We also coexist with external organisms in our environment.
  • Infection occurs when this balance is disrupted, either by an excess of external organisms or imbalances in normal flora.

Types of Pathogens

  • Always pathogenic: Even a few organisms can cause illness.
  • Potential pathogens: Cause illness when established and colonized.
  • Process of infection:
    • Penetration of defenses.
    • Invasion of tissues.
    • Multiplication.
    • Production of disease features (clinical signs and symptoms).

Definition of Infectious Disease

  • Harmful alteration of physiology or metabolism caused by a microorganism or its products.
  • Caused by a pathogen.

Relationships with Microorganisms

  • Symbiotic relationship: Mutually beneficial (e.g., bacteria in a cow's gut aiding nutrient absorption from grass).
  • Commensalism: One organism benefits, the other is not harmed (e.g., normal flora consuming nutrients on our body without harming us).
  • Opportunism: Commensal bacteria causing infection when the opportunity arises.
  • Parasitism: One organism benefits at the expense of the other; can range in severity:
    • Minor: Candida albicans causing thrush, athlete’s foot.
    • Moderate: Salmonella causing gastroenteritis.
    • Severe: Vibrio cholerae causing severe illness and potentially death.

Types of Infections

  • Endogenous Infections: Arise from bacteria living on/in the body; normal flora moves to an unusual location.
    • Examples:
      • Staphylococcus aureus on the skin entering a cut and causing a wound infection.
      • Escherichia coli moving from the bowel to the urethra, causing a urinary tract infection.
    • Latent viral infections may also be classified this way.
  • Congenital Infections: Passed from mother to baby in utero or during birth.
    • Examples:
      • In utero: Rubella, cytomegalovirus, and toxoplasmosis crossing the placenta.
      • During birth: Hepatitis B and HIV transmitted via mother's blood.
  • Exogenous Infections: Acquired from the environment.
    • Examples: COVID-19, Vibrio cholerae, Clostridium botulinum.
    • Healthcare-associated infections (HAIs):
      • Also known as iatrogenic, hospital-acquired, or nosocomial infections.
      • Occur in hospitals.
      • Often involve antibiotic-resistant organisms.
      • Patients are more susceptible due to weakened immune systems.
    • Most exogenous infections occur in the community.