Microbial Mechanisms of Pathogenicity Notes

Learning Objectives

  • Distinguish between signs and symptoms of disease.
  • Explain the difference between communicable diseases and noncommunicable diseases.
  • Identify and describe the stages of an acute infectious disease in terms of the number of pathogens present and the severity of signs and symptoms.
  • Explain the concept of pathogenicity (virulence) in terms of infectious and lethal dose.
  • Distinguish between primary and opportunistic pathogens and identify specific examples of each.
  • Summarize the stages of pathogenesis.
  • Explain how virulence factors contribute to signs and symptoms of infectious diseases.
  • Differentiate between endotoxins and exotoxins.
  • Describe virulence factors unique to fungi and parasites.
  • Compare virulence factors of fungi and bacteria.

Signs and Symptoms of Disease

  • Signs: Objective and measurable indicators of disease observable by clinicians (e.g., fever, blood pressure).
  • Symptoms: Subjective feelings or experiences reported by patients (e.g., pain, fatigue).
Clinical Example
  • Michael's Case: 10-year-old boy experiences fever (38 °C) and nausea after eating an undercooked hot dog. Signs progress to severe headaches and high fever (40 °C).

Definition of Disease

  • A condition where normal body structures or functions are damaged or impaired (e.g., infection, genetics, environmental factors, immune response).
Infection
  • Infection: Successful colonization of a host by a microorganism, which can produce signs and symptoms of disease.
  • Pathogens: Microorganisms that cause disease.

Classification of Diseases

1. Infectious Diseases
  • Caused by pathogens (both cellular - bacteria, parasites, fungi; and acellular - viruses, prions).
  • Communicable Diseases: Spread from person to person through direct or indirect means.
  • Contagious Diseases: Easily spread; contagion levels can vary.
2. Non-Communicable Infectious Diseases
  • Not transmitted person-to-person (e.g., tetanus).
3. Noninfectious Diseases
  • Caused by factors like genetics or environment (e.g., sickle cell anemia).
Types of Noninfectious Diseases
  • Inherited: Genetic diseases (e.g., sickle cell anemia).
  • Congenital: Present at birth (e.g., Down syndrome).
  • Degenerative: Progressive loss of function (e.g., Parkinson's disease).
  • Nutritional Deficiency: Impairment due to nutrient lack (e.g., scurvy).
  • Endocrine: Hormonal malfunctions (e.g., hypothyroidism).
  • Neoplastic: Abnormal growth (e.g., cancers).
  • Idiopathic: Causes unknown.

Stages of Infectious Diseases

  1. Incubation Period: Time after pathogen entry without signs/symptoms.
  2. Prodromal Period: General signs/symptoms indicating illness.
  3. Period of Illness: Most severe symptoms present.
  4. Period of Decline: Signs/symptoms decrease; risk of secondary infection.
  5. Period of Convalescence: Return to normal functions; possible permanent damage.

Pathogenicity and Virulence

  • Pathogenicity: Ability of a microbe to cause disease.
  • Virulence: Degree of pathogenicity, determined by the median infectious dose (ID50) and median lethal dose (LD50).

Primary vs. Opportunistic Pathogens

  • Primary Pathogens: Cause disease regardless of host (e.g., Enterohemorrhagic E. coli).
  • Opportunistic Pathogens: Cause disease when host defenses are compromised (e.g., Candida species, Staphylococcus epidermidis).

Stages of Pathogenesis

  1. Exposure: Contact with potential pathogens.
  2. Adhesion: Microbes attach to host cells using adhesins (e.g., fimbriae, biofilms).
  3. Invasion: Pathogen spreads, often using toxins or enzymes.
  4. Infection: Local, focal, or systemic, depending on the extent of dissemination.
Portals of Entry/Exit
  • Pathogens access hosts via mucosal surfaces, skin breaches, and through portals like the respiratory tract.
  • Exit through skin, respiratory, urogenital, gastrointestinal tracts, and vectors.

Virulence Factors in Specific Pathogens

Bacterial Adhesins
  • Example: Streptococcus pyogenes (Protein F) adheres to respiratory cells.
Viral Adhesins
  • Example: Influenza hemagglutinin binds to respiratory epithelial cells.
Fungal Virulence Factors
  • Candida albicans: Produces adhesins and enzymes (proteases, phospholipases) for tissue invasion.
Protozoan and Helminth Virulence Factors
  • Protozoans: Use specialized adhesion mechanisms and can cause inflammation.
  • Helminths: Utilize proteases for skin penetration and have defenses against the immune response.

Clinical Focus - Michael's Case

  • Symptoms lead to suspected meningitis. A spinal tap confirms infection with aggressive antibiotic therapy prescribed.

Conclusion

  • Understanding microbial pathogenicity and disease mechanisms is crucial for diagnosis and treatment of infectious diseases.