Overview of Infection and Disease Part B

Overview of Infection and Disease

  • Difference Between Infection and Disease

    • Microorganisms can be present in a host without causing disease.

Steps for Microorganisms to Cause Disease

  1. Entrance (Portal of Entry)

    • Microorganisms must gain entrance into the organism.

    • Portal of Entry Definition: An opening in the skin or into the body where the microorganism can enter.

    • Examples of portals of entry:

      • Eyes

      • Respiratory tract

      • Gastrointestinal tract

      • Genitourinary tract

      • Skin openings (pores, hair follicles, cuts, bites, blisters)

    • Importance of Portal of Entry:

      • Different organisms have specific portals of entry that determine whether they can cause disease.

      • Example: E. coli on skin may not cause disease, but ingested E. coli can indeed cause harm.

  2. Adherence to Tissue

    • After entering, microorganisms must adhere to tissues to remain inside long enough to cause disease.

    • Mechanisms of Adherence:

      • Fimbriae: Short hair-like extensions on the organism's surface that allow sticking to surfaces.

      • Hooks: Some organisms can bore into tissue, e.g., Treponema pallidum (syphilis).

      • Capsule/Glycocalyx: A sticky layer that helps the organism attach and resist removal.

  3. Surviving the Host's Defenses

    • After adherence, pathogens must evade the immune response:

      • Preventing Phagocytosis:

        • Some pathogens produce leukocidins, toxins that destroy white blood cells, forming a clear zone around colonies on agar.

      • Slime Layer/Capsule: Inhibits phagocyte engulfment.

      • Intracellular Survival: Some pathogens (e.g., Mycobacterium tuberculosis, Mycobacterium leprae) can survive inside phagocytes.

      • Antigen Variation: Pathogens can change surface proteins (antigens) (e.g., influenza virus changes hemagglutinin and neuraminidase) to escape recognition by the immune system.

  4. Causing Disease

    • Pathogens can cause disease through various mechanisms:

      • Toxins: Production of exotoxins and endotoxins.

        • Exotoxin:

        • Secreted during cell growth.

        • Typically proteins/enzyme that can cause disease in very small amounts.

        • Examples: Botulinum toxin (causes paralysis), tetanus toxin.

        • Mostly produced by Gram-positive organisms.

        • Endotoxin:

        • Part of the outer membrane of Gram-negative bacteria.

        • Released upon cell death.

        • Causes similar symptoms regardless of source (fever, inflammation, hemorrhage, diarrhea).

        • Requires a higher dose to cause harm compared to exotoxins.

      • Host Defense Damage: Damage can also be inflicted by the host's own immune response trying to eliminate the pathogen.

      • Viral Pathogenesis: Certain viruses can induce changes in host DNA, potentially leading to diseases like cancer.

  5. Transmission to New Hosts

    • Organisms must exit the host to find and infect new hosts, with methods including:

      • Coughing and sneezing

      • Skin lesions (exiting through skin cells)

      • Urine or feces

      • Blood

      • Insect bites (vector transmission)

Summary of Key Statistics

  • ID50 (Infectious Dose 50):

    • Number of pathogen cells required to infect 50% of a population.

    • Lower ID50 indicates higher pathogenicity.

  • LD50 (Lethal Dose 50):

    • Number of pathogen cells needed to kill 50% of a population.

    • Lower LD50 indicates higher lethality.

Conclusion

  • The cycle of infection involves multiple steps: entering the body, adhering, surviving immune defenses, causing disease, and transmitting to new hosts. Understanding each step is essential for studying infectious diseases and developing control strategies.