Microbial Mechanisms of Pathogenicity Notes

Disease Definition and Classifications

  • Disease: Any condition impairing normal body structure/function; includes infections, genetic issues, and environmental causes but excludes physical injuries.

  • Communicable Diseases: Spread from person to person (e.g., influenza).

  • Noncommunicable Diseases: Not spreadable (e.g., tetanus).

  • Iatrogenic Diseases: Caused by medical treatment (e.g., infections from catheter use).

  • Zoonotic Diseases: Transmitted from animals to humans (e.g., rabies).

Stages of Infectious Diseases

  1. Incubation Period: Pathogen entry; asymptomatic development.

  2. Prodromal Period: General signs emerge; non-specific symptoms like fever.

  3. Period of Illness: Severe symptoms; disease is most apparent.

  4. Period of Decline: Pathogen numbers decrease; patient at risk for secondary infections.

  5. Period of Convalescence: Recovery phase.

Pathogenicity and Virulence

  • Pathogenicity: Ability of microbes to cause disease.

  • Virulence: Degree of pathogenicity based on infectious and lethal doses:

    • Infectious Dose (ID50): Number of pathogens needed to infect 50% of a population.

    • Lethal Dose (LD50): Amount of toxin needed to kill 50% of a population.

  • Example distinctions: Highly virulent pathogens cause disease consistently; less virulent may cause mild symptoms or be asymptomatic.

Primary vs. Opportunistic Pathogens

  • Primary Pathogens: Cause disease in healthy hosts (e.g., E. Coli).

  • Opportunistic Pathogens: Cause disease when host defenses are compromised (e.g., Candida species, Staphylococcus epidermidis).

Stages of Pathogenesis

  1. Exposure: Contact with a pathogen from food, surfaces, etc.

  2. Adhesion: Pathogen attaches to host cells using adhesins (proteins on pathogens).

  3. Invasion: Spread of pathogen within host tissues.

  4. Infection: Can be local (confined to area), focal (spreading), or systemic (widespread).

Unique Virulence Factors

  • Fungal Virulence:

    • Candida albicans: Adhesins and enzymes (proteases, phospholipases) promote tissue invasion.

    • Cryptococcus: Capsule production enhances resistance to the immune system.

  • Helminth Virulence: Utilize proteases for skin penetration; possess outer cuticle for immune evasion.

  • Protozoan Virulence: Use unique adhesion mechanisms (e.g., Giardia), causing inflammation rather than invasive infections.

Bacterial Pathogenicity: Secrete exotoxins to disrupt host cell functions and can form biofilms to enhance survival in hostile environments.