Micro 4: Pathogenesis of Bacterial Infection Study Notes

Pathogenesis of Bacterial Infection

Course Information

  • Class: Year 1

  • Course: Undergraduate Medicine

  • Lecturer: Dr Ellen Browne

  • Date: 21st October 2025

Learning Outcomes

  1. Differentiate the normal flora (microbiome) from virulent & opportunistic pathogens.

  2. Explain how pathogens are transmitted.

  3. Describe how pathogens gain entry into the body and cells.

  4. Explain how pathogens avoid being killed by the immune system.

  5. Describe how pathogens spread in the body.

  6. Differentiate between endotoxins and exotoxins.

  7. Discuss host risk factors for infection.

Pathogenesis Definition

  • Pathogenesis: The mechanisms involved in the production of disease.

Key Questions

  • How can something so small cause so much trouble?

    • Detected using microscope settings (30.0 kV, Acc.V Spot Magn 20000x).

    • Example: Image sourced from DermNetNZ.org illustrating Staphylococcus aureus.

Definitions

  • Pathogen: A microorganism that is able to cause disease (pathology).

  • Pathogenicity: The ability to produce disease in a host.

  • Virulence: The capacity/severity of a microorganism to cause disease or to invade.

Pathogenicity vs Virulence

  • Pathogenicity: Relates to the organism's ability to cause disease.

  • Virulence: Refers to the degree of pathology caused by the microorganism.

Microbial Disease Statistics

  • What percentage of bacteria can cause disease in humans?

    • A. < 1%

    • B. 5%

    • C. 10%

    • D. 25%

    • Source: Microbiology Society

Human Microbiome Overview

  • Visible Organs vs Invisible Microbiome:

    • The human body hosts approximately 10^{14} cells and 10^{14} million microbes.

    • The microbiome contributes significantly to bodily functions, aiding in digestion, immune defense, and more.

Microbiome Functions

  • Nose: Mucus production, antimicrobial chemicals.

  • Mouth: Assists with digestion and wards off pathogens.

  • Lungs: Lubricates pulmonary tissues.

  • Stomach: Prevents gastric complications.

  • Colon: Digestion of complex carbohydrates, maintenance of pH, H₂O₂ production.

  • Skin: Fortifies the immune system, scent production.

Normal Flora (Microbiome)

  • Commonly non-pathogenic organisms present on or in our bodies without causing disease.

  • Can cause infections if:

    • They escape from their usual location.

    • There is a breach in barriers.

    • The host is immunosuppressed.

Virulent Bacteria

  • Defined by their increased ability to invade or damage the host.

  • Primary Pathogens: These are capable of causing disease simply by their presence in a host. They can live among normal flora without causing harm and are considered "colonized."

  • Virulence Factors: Genetic, biochemical, or structural features that enhance a pathogen's ability to cause disease.

Methicillin Resistant Staphylococcus Aureus (MRSA)

  • An example of a virulent bacteria showing the transition from colonization to infection.

Opportunistic Bacteria

  • Have low intrinsic virulence and typically do not cause infections in immunocompetent hosts.

  • May cause serious infections under specific circumstances (e.g., immunocompromised individuals or when a foreign body is present).

Factors for Infection Occurrence

  1. The organism is virulent.

  2. The size of the inoculum (the quantity of pathogen present).

  3. The portal of entry (pathways the pathogen uses to enter).

  4. Survival in the host.

  5. The state of the host’s immune system.

Pathogen Transmission

  • From the Pathogen’s Perspective:

    1. Get in (Portal of Entry).

    2. Attach to cells.

    3. Defeat/evade the immune system.

    4. Cause damage to host cells.

    5. Get out and spread further.

Portals of Entry for Pathogens

  1. Ingestion: Example - Salmonella food poisoning.

  2. Inhalation: Example - Tuberculosis.

  3. Penetration/Inoculation: Example - Staphylococcus aureus.

  4. Sexual Transmission: Example - Chlamydia.

  5. Vertical Transmission: Example - Group B Streptococcus.

Mechanisms of Entry and Immune Evasion

  • Adhesion: Required for establishment within the host.

    • Prevents bacterial clearance from mechanical host defenses and occurs through receptor/ligand mediated processes (Adhesins).

Biofilm Formation

  • Adhesion of bacteria to the surface of foreign materials in the body, for instance, joint replacements.

  • Biofilms protect bacteria, making eradication difficult unless the foreign material is removed.

Invasion Mechanisms

  • Pathogens employ biochemical virulence factors known as **“Invasins.