LESSON-7-HOST-DEFENSE-AND-PHATOGENICITY

Pathogenesis II: Host Defense and Pathogenicity of the Microbes in Microbiology

Overview

  • Study of host defense mechanisms against infectious diseases.

  • Examination of microbial pathogenicity.

Outline of Host Defense Against Infectious Disease

  • Define Normal Flora

  • Distribution of Normal Flora

  • Protective function and clinical significance of Normal Flora

  • Stages of Bacterial Pathogenesis

  • Mode of Transmission

  • Routes of Transmission

Mechanisms of Host Defense

Innate Immunity

  • Physical Barriers

    • Skin: Keratin layer, fatty acids from sebaceous glands, low pH, support from normal flora.

    • Mucous Membranes: Lines respiratory and gastrointestinal tracts.

    • Phagocytic Cells: Including macrophages and polymorphonuclear leukocytes (PMN).

Soluble Molecules

  • Complement: Set of proteins aiding in immune response.

  • Lysozyme: Enzyme that breaks down bacterial cell walls.

  • Antibodies: Produced by B cells.

  • T Lymphocytes: Key players in adaptive immunity.

Physical Barriers

  • Skin: Important for preventing pathogen entry.

  • Respiratory Tract: Mucus lining, IgA presence, ciliary action, and alveolar macrophages to trap pathogens.

  • Gastrointestinal Tract: Acidic stomach pH and digestive enzymes protect against ingested pathogens.

  • Vagina: Maintains low pH through Lactobacilli bacteria.

Factors Contributing to Bacterial Infection

Impact of Barrier Damage

  • Intravenous Catheter: May lead to skin infections like Staphylococcus epidermidis and S. aureus.

  • Diabetes: Higher risk for skin infections by S. aureus.

  • Burns: Vulnerability to infections from Pseudomonas aeruginosa.

  • Dental Extraction: Can lead to infections from Streptococcus viridans due to disruptions in mucosal integrity.

Host Defense Mechanisms

Inflammation

  • Phagocytosis as a response to infection.

Inflammatory Response Types

  • Pyogenic Response: Leads to pus formation; involves acute, extracellular pathogens and activated by complements and antibodies.

  • Granulomatous Response: Formation of granulomas; involves chronic responses with macrophages and helper T cells against intracellular pathogens.

Complement Activation Pathways

  • Classical, Alternative, and Lectin Pathways: Various mechanisms for complement activation that enhances immune response.

  • Involvement of C3 and C5 convertases in amplification and facilitation of pathogen opsonization and lysis.

Inflammatory Response Against Bacterial Infection

  • Responses differ for pyogenic versus intracellular bacteria.

  • Activation leads to mediator release (e.g., histamine) that increases vascular permeability, causing edema and erythema.

Phagocytosis Process

Steps in Phagocytosis

  1. Migration: Mediated by IL-8 and C5a; adhesion mechanisms via selectins and integrins.

  2. Ingestion: Phagosome formation, aided by opsonization (IgG, C3b).

  3. Killing: Mechanisms deployed to destroy ingested pathogens.

Normal Flora and its Role in Host Defense

Definition

  • Normal Flora: Permanent residents of body sites (e.g., skin, colon).

Commensals

  • Organisms that benefit from the host without causing harm.

Significance of Normal Flora

  • Disease potential when immune system compromised or when organisms move to unusual sites.

  • Important for colonization resistance; limits pathogenic bacteria by occupying binding sites.

  • Metabolic Functions: Synthesis of vitamins like B12 and K.

Normal Flora Distribution by Body Sites

  • Skin: Includes species like Staphylococcus epidermidis, Propionibacterium acnes.

  • Respiratory Tract: Different flora in nose and throat (e.g., Staphylococcus aureus, Viridans streptococci).

  • Intestinal Tract: Presence of E. coli and various lactobacilli; yeasts like Candida albicans.

  • Genitourinary Tract: Includes Group B Streptococci and Candida albicans.

Stages of Bacterial Pathogenesis

  1. Transmission: Entry through the portal.

  2. Evasion: Overcoming host defenses.

  3. Adherence: Attachment to host tissues.

  4. Colonization: Establishing presence.

  5. Disease: Resulting from toxin production or invasion.

  6. Host Responses: Reactions from the immune system.

  7. Progression or Resolution: Outcome of the infection.

Summary of Key Points

  • Host defense involves innate and acquired immunity mechanisms; macrophages and T-cells control intracellular pathogens.

  • Complement, antibodies, and inflammatory responses play pivotal roles in combating extracellular pathogens.

  • Disease types vary; some involve toxin production, while others are related to invasive mechanisms.

  • Normal flora function as both protective agents and potential pathogens under specific conditions.

References

  • Levinson, W. (2010). Review of Medical Microbiology and Immunology, 11th ed.

  • Brooks, G. F., Carroll, K. C., Butel, J. S., Morse, S. A., Mietzner, T. A. (2010). Medical Microbiology, 25th ed.