NURS 232 Mechanisms of Infectious Disease

Mechanisms of Infectious Disease

Key Components of Infection

1. BUG: The infectious agent (e.g., bacteria, virus, fungus, parasite).

2. HOST: The organism that is infected.

3. PROTEINS: Involved in infection processes (e.g., toxins, adhesion factors).

Infection Dynamics

Incubation Period

• The time from exposure to the pathogen until symptoms appear.

• Can range from hours (e.g., food poisoning) to years (e.g., HIV).

Infection vs. Colonization

• Colonization: Pathogen is present but not causing harm (e.g., normal gut bacteria).

• Infection: Pathogen is present and causing disease.

Microflora vs. Pathogens

• Microflora: Normal microorganisms living in/on the body. Most are beneficial or neutral.

• Pathogens: Microorganisms that cause disease:

  • Opportunistic pathogens: Cause disease when the host’s immunity is compromised (e.g., S. aureus, C. difficile).

Types of Infectious Agents

Prions

• Unique infectious agents: Made of abnormal proteins (no DNA/RNA).

• Cause conformational changes in normal proteins, leading to cell damage.

• Examples: Mad cow disease, Creutzfeldt-Jakob disease.

Viruses

• Structure: Protein coat (capsid) surrounding a DNA or RNA core.

• Replication: Requires a host cell to reproduce.

• Examples:

  • Rhinovirus (30 nm): Common cold.

  • Ebola virus (970 nm): Severe hemorrhagic fever.

  • SARS-CoV-2: Binds to ACE-2 receptors in the lungs.

Bacteria

• Prokaryotic cells: No nucleus or membrane-bound organelles.

• Structures:

  • Single chromosome, plasmids, cell wall, cytoplasmic membrane.

  • Can form biofilms (protective communities).

• Examples:

  • Helicobacter pylori: Causes stomach ulcers.

  • Mycoplasmas: Smaller bacteria, similar to viruses.

Fungi

• Prefer cooler temperatures than the human body.

• Typically infect surface tissues (e.g., skin, nails).

• Examples: Candida (yeast infections), Tinea (ringworm).

Parasites

1. Protozoa: Single-celled organisms (e.g., Plasmodium causing malaria).

2. Helminths: Worms (e.g., roundworms, tapeworms).

3. Arthropods: Vectors like ticks and mosquitoes.

How Pathogens Enter the Body (Portals of Entry)

1. Penetration: Breaks in the skin (e.g., cuts, bites).

2. Direct Contact: Close physical interaction (e.g., STIs).

3. Ingestion: Contaminated food or water (e.g., E. coli).

4. Inhalation: Breathing in pathogens (e.g., influenza, tuberculosis).

Virulence Factors

• Components that help pathogens cause disease:

  1. Toxins:

     • Exotoxins: Secreted by bacteria (e.g., tetanus toxin).

     • Endotoxins: Part of bacterial structure (e.g., LPS in Gram-negative bacteria).

  2. Adhesion Factors: Help pathogens stick to host tissues.

  3. Evasive Factors: Help pathogens avoid the immune system.

Antibacterial vs. Antiviral Drugs

Antibacterial Drugs

• Targets:

  • Cell wall synthesis (e.g., penicillin).

  • Protein synthesis (e.g., tetracyclines).

  • Nucleic acid synthesis (e.g., quinolones).

• Bacterial Resistance:

  • Alter drug targets.

  • Use different metabolic pathways.

  • Modify cell walls to block drug entry.

Antiviral Drugs

• Targets:

  • Viral RNA/DNA synthesis.

  • Viral enzymes (e.g., reverse transcriptase in HIV).

  • Viral attachment to host cells.

• Challenges:

  • Viruses use host cell machinery, making it harder to target them without harming the host.

Case Study: Helicobacter pylori

• Scenario: A patient with H. pylori infection develops a bleeding ulcer.

• Mechanism:

  • H. pylori survives in the acidic stomach environment.

  • Produces toxins that damage the stomach lining.

  • Stress and other factors exacerbate the damage, leading to ulcers.

Key Takeaways

1. Pathogens include bacteria, viruses, fungi, parasites, and prions.

2. Virulence factors (e.g., toxins, adhesion factors) determine how pathogens cause disease.

3. Antibacterial drugs target bacterial structures, while antiviral drugs target viral replication.

4. Portals of entry (e.g., skin, inhalation) determine how infections start.