Bacteria and Disease Study Notes

Bacteria and Disease Study Notes

Learning Objectives

At the end of this chapter, the student should be able to:

  1. Define common terms involved in the production of disease by bacteria.
  2. Explain the components of Koch's postulates.
  3. Discuss thoroughly the various factors that play a role in the chain of infection.
  4. Compare the various mechanisms by which bacteria produce disease, citing examples for each mechanism.
  5. Describe the various ways by which infectious diseases are classified.
  6. Compare the events involved in the various stages of an infectious disease.

Definitions of Disease Types

  • Sporadic Disease: Occurs occasionally in a population.
  • Endemic Disease: Constantly present in a population at low levels (e.g., malaria in Palawan, schistosomiasis in Leyte).
  • Epidemic Disease: Affects a large number of people in a locality in a short period (e.g., influenza).
  • Pandemic Disease: Worldwide occurrence affecting at least three regions (e.g., influenza A, SARS, AIDS).

Severity and Duration of Diseases

  • Acute Disease: Develops rapidly, lasts a short period (e.g., common cold).
  • Chronic Disease: Develops slowly, lasts long (e.g., tuberculosis, hepatitis B which can be acute or chronic).
  • Latent Disease: Causative organism is inactive but can activate later (e.g., shingles reactivating from chickenpox).

Extent of Host Involvement

  1. Localized Infection: Invading organisms limited to a small area (e.g., boils, abscesses).
  2. Systemic Infection: Causative organisms spread throughout the body through blood or lymph.
  3. Focal Infection: Localized infection that spreads through blood or lymph to specific areas (e.g., dental abscess).
  4. Primary Infection: Initial acute infection.
  5. Secondary Infection: Caused by opportunistic pathogens following primary infection; often more severe.
  6. Subclinical/Inapparent Infection: Does not cause noticeable illness (e.g., hepatitis A may not show symptoms).

Stages of an Infectious Disease

  1. Incubation Period: Time between entry of the pathogen and appearance of symptoms. This period varies (e.g., 6-12 days) and is influenced by:

    • Virulence: Stronger pathogens can cause disease faster.
    • Number of Microorganisms: More microorganisms can reduce incubation period.
    • Host Resistance: Stronger immune responses may lengthen incubation.

  2. Prodromal Period: Mild, non-specific symptoms appear (e.g., fever, malaise). Not disease-specific.

  3. Period of Illness:

    • Most severe symptoms occur, maximal invasion of pathogen.
    • For example, measles causes rashes and increased WBC counts (elevated neutrophils in bacterial infections, elevated lymphocytes in viral infections).
    • Possible outcomes:
      • Recovery and resolution of infection.
      • Progression to severe disease (fulminant infection).
      • Shift to chronic form (e.g., chronic hepatitis).
      • Carrier state where the patient is asymptomatic but can transmit infection.

  4. Period of Decline: Signs and symptoms decrease, recovery begins; patient becomes susceptible to secondary infections.

  5. Period of Convalescence: Recovery occurs, patient regains strength, returns to normal condition.

Koch's Postulates

  • Developed by Robert Koch to establish a causal relationship between a microbe and a disease. The postulates are:
    1. The pathogen must be present in diseased individuals but absent in healthy ones.
    2. Pathogen must be isolated from the diseased host and grown in pure culture.
    3. Pathogen grown in culture must cause the disease when introduced to a healthy, susceptible individual.
    4. Same pathogen must be re-isolated from the inoculated, diseased individual.
  • Limitations:
    • Some pathogens can't be cultured in vitro (e.g., viruses, Mycobacterium leprae).
    • Some infections remain subclinical, affecting the applicability of the postulates.
    • Ethical considerations when using human subjects for testing.
    • Certain pathogens may change behavior when cultured.

Factors Influencing Infection

  • Chain of Infection: Interaction between the etiological agent, the host, and the environment.
    • Reservoirs: Sites where infectious agents live and multiply (e.g., animals, humans, environments). Includes
      • Animal Reservoirs: Zoonotic infections (e.g., anthrax, plague).
      • Human Reservoirs: Can transmit directly or harbor asymptomatically.
    • Portal of Exit: Route of exit from the reservoir, which can be the same as the entry route for another host.
    • Mode of Transmission: Ways pathogens are transmitted (e.g., direct contact, airborne transmission).
    • Portal of Entry: Route through which the pathogen enters a susceptible host (often the same as the exit).
    • Susceptible Host: Factors affecting susceptibility include genetic factors and immune status.

Mechanisms of Disease Production

Mechanical: Invasiveness
  • Direct damage/multiplication of pathogens in host tissues:
    1. Colonization: Entering and establishing in the host.
    2. Evasion of Immune Defenses: Capsulated bacteria can evade phagocytosis or survive in macrophages.
    • Example: Staphylococcus aureus.
    1. Extracellular Production of Substances: Promote invasion (e.g., Neisseria gonorrheae).
    2. Enzymatic Activities: Break down host tissues (e.g., collagenase by Clostridium perfringens causes gas gangrene).
Chemical: Toxin Production
  • Toxins are poisonous substances causing disease.
    • Endotoxins: Integral components of gram-negative bacteria (e.g., LPS from Salmonella, E. coli).
    • Released upon lysis, similar effects regardless of bacteria.
    • Exotoxins: Released proteins from living bacteria affecting specific host functions.
    • Types include cytotoxins (kill cells), neurotoxins (nerve function interference), enterotoxins (gastrointestinal tract).
Immunologic Response
  • Damage may arise from the immune response to pathogens (not directly due to microorganisms).
    • Examples include autoimmune responses or cytotoxic T-cell response in viral infections.

Classification of Infectious Diseases

By Behavior in Hosts
  • Communicable Disease: Spread from host to host (e.g., measles).
  • Contagious Disease: Easily transmitted (e.g., measles, chickenpox).
  • Fulminant Infection: Rapid onset; severe (e.g., meningococcemia).
  • Non-communicable Disease: Not transmitted between hosts (e.g., tetanus).
By Source
  • Exogenous Infection: Outside source (e.g., cholera from contaminated water).
  • Endogenous Infection: Source from within (e.g., E. coli leading to UTIs).
By Occurrence
  • Sporadic Disease: Occasional occurrence.
  • Endemic Disease: Constantly low-level presence.
  • Epidemic Disease: High levels in a locality.
  • Pandemic Disease: Global impact.
By Severity or Duration
  • Acute, chronic, and latent infections defined by duration and severity.