Microbial Mechanisms of Pathogenicity Notes

Microbial Mechanisms of Pathogenicity - Chapter 15

15.1 Characteristics of Infectious Disease

  • Disease: Condition where normal structure and/or function are damaged or impaired.

  • Infection: Invasion of a pathogen or parasite that leads to disease.

    • Signs: Measurable indicators (e.g., blood cell counts).

    • Symptoms: Subjective feelings experienced by the patient (e.g., nausea).

  • Syndrome: A combination of signs and symptoms indicating a particular disease.

    • Helps in directing towards the diagnosis of a disease.

    • Patients may be asymptomatic (only signs observable through testing).

Nomenclature of Symptoms

  • Prefix Composition:

    • cyto-: cell

    • hepat-: of the liver

    • -pathy: disease

    • -emia: of the blood

    • -itis: inflammation

    • -lysis: destruction

    • -oma: tumor

    • -osis: diseased/abnormal condition

    • -derma: of the skin

Classification of Disease

  • WHO's International Classification of Diseases (ICD) is used globally.

  • Types of Disease:

    • Infectious: Caused by pathogens.

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

    • Iatrogenic: Acquired as a result of a medical procedure.

    • Nosocomial: Acquired from a hospital setting.

    • Zoonotic: Acquired from animals.

    • Non-communicable: Obtained from non-living entities (e.g., soil).

    • Non-infectious: Not caused by pathogens.

Types of Noninfectious Diseases

  • Inherited: Genetic diseases (e.g., sickle cell anemia).

  • Congenital: Present at or before birth (e.g., Down syndrome).

  • Degenerative: Progressive loss of function (e.g., Parkinson's disease).

  • Nutritional: Deficiency-related diseases (e.g., scurvy).

  • Endocrine: Gland malfunction diseases (e.g., hypothyroidism).

  • Neoplastic: Abnormal growth (e.g., certain cancers).

  • Idiopathic: Unknown cause diseases.

Communicable vs. Non-communicable Diseases

  • Example:

    • Communicable: Malaria

    • Non-communicable: Sickle Cell Anemia

Periods/Stages of Disease

  • Infectious diseases follow five stages:

    1. Incubation: Initial entry of the pathogen and replication.

    2. Prodromal: Signs and symptoms begin to appear.

    3. Illness: Most severe signs and symptoms.

    4. Decline: Pathogen numbers decrease but risk of secondary infections rises.

    5. Convalescence: Recovery begins.

Acute vs. Chronic Disease

  • Acute Disease: Short duration (hours to weeks).

  • Chronic Disease: Longer duration (months, years, or lifetime).

  • Latent Disease: Episodes of active replication followed by dormant phases.

15.2 How Pathogens Cause Disease

Koch’s Postulates

  • Set standards to demonstrate that a specific pathogen causes a specific disease:
    1) Pathogen must be present in diseased organisms, absent in healthy ones.
    2) Pathogen must be isolated from diseased organism and grown in pure culture.
    3) Cultured agent must cause the same disease when inoculated into a healthy organism.
    4) Same pathogen must be reisolated from the inoculated organism.

Molecular Koch’s Postulates

  • Improved with molecular methodologies, identifying genes instead of pathogens.

  • Limitations include difficulty in genetic manipulation and lack of suitable animal models for some diseases.

Pathogenicity and Virulence

  • Pathogenicity: Ability of pathogen to cause disease.

  • Virulence: Degree of pathogenicity (e.g., high virulence: Bacillus anthracis; low virulence: Rhinovirus).

  • Virulence Metrics:

    • ID50: Number of pathogens required to infect 50% of exposed individuals.

    • LD50: Number of pathogens required to kill 50% of infected individuals.

Primary vs. Opportunistic Pathogens

  • Primary Pathogen: Causes disease regardless of host defenses (e.g., enterohemorrhagic E. coli).

  • Opportunistic Pathogen: Causes disease only in compromised hosts (e.g., Candida albicans in disrupted microbiota).

Stages of Pathogenicity

  1. Exposure: Contact with the host.

  2. Adhesion: Pathogen adheres to host cells.

  3. Invasion: Pathogen invades tissues.

  4. Infection: Establishes infection.

  5. Transmission: Spread to new hosts.

Pathogens and Portals of Entry

  • TORCH Infections: Pathogens capable of crossing the placental barrier (e.g., Toxoplasmosis, Rubella).

Mechanisms of Invasion

  • Adhesion Factors:

    • Adhesins: Bind to host receptors to establish infection.

    • Biofilms: Grouping of microorganisms that enhances resistance to immune responses.

  • Invasion Mechanisms:

    • Effector Proteins and Surface Proteins allow entry and colonization.

    • Some intracellular pathogens can survive within immune cells.

Infection Types

  • Local Infection: Restricted to a small area.

  • Focal Infection: Spreads to secondary sites.

  • Systemic Infection: Affects the whole body (e.g., septicemia).

15.3 Virulence Factors of Bacterial and Viral Pathogens

Virulence Factors Overview

  • Virulence Factors: Assist pathogens in infection and disease severity.

    • Types include adhesion factors, exoenzymes, toxins, and immune evasion mechanisms.

Adhesins

  • Adhesins: Proteins that facilitate attachment to host cells.

  • Found in various microbes (bacterial, viral, fungal).

  • Facilitate biofilm formation.

  • Examples include Protein F in Streptococcus pyogenes (attaches to respiratory epithelial cells) and Type IV pili in Neisseria gonorrhoeae (attach in gonorrhea).

Toxins

  • Endotoxins: Trigger inflammatory responses, can cause severe fever and shock.

  • Exotoxins: Proteins targeting specific cell receptors.

    • Types of Exotoxins:

    • Intracellular targeting (e.g., diphtheria toxin).

    • Membrane-disrupting toxins (e.g., phospholipases).

    • Superantigens that trigger excessive cytokine production.

15.4 Virulence Factors of Eukaryotic Pathogens

Fungi

  • Similar properties to bacteria (adhesins, exoenzymes, toxins).

  • E.g., Cryptococcus spp. has a capsule, Mycotoxins from Claviceps and Aspergillus affect crops.

Protozoans

  • Use similar virulence factors (adhesins, toxins).

  • Unique features like Giardia lamblia’s adhesive disk attach to intestines.

Helminths

  • Often penetrate tissue using proteases, produce structures like cuticles to withstand host defenses, and mimic host cells to evade immune detection.

These notes provide an organized overview of the key concepts related to microbial mechanisms of pathogenicity as detailed in the transcript. They can serve as a solid foundation for exam preparation.