Host-Microbe Interactions and Pathogenesis

Normal Microbiota and Dysbiosis

The human body is colonized by a diverse community of microorganisms known as the normal microbiota, which inhabit the skin, digestive, genital, urinary, and respiratory systems. This relationship is typically mutualistic, providing benefits to both the host and the microbes.

• Benefits of Normal Microbiota:

  • Manufacture essential vitamins (e.g., vitamin K, B vitamins)

  • Compete with potential pathogens for resources and space

  • Promote maturation of the immune system

• Dysbiosis: Disruption of the normal microbiota, often due to antibiotics, can compromise health and allow opportunistic pathogens (e.g., Candida albicans) to overgrow.

• Host Factors: Differences in age, gender, health, and habits can influence whether a microbe is harmless or pathogenic in a given individual.

• Example: Group B Streptococci (GBS) are normal commensals in ~30% of women but can cause sepsis, meningitis, and pneumonia in newborns.

Pathogenicity, Virulence, and Virulence Factors

Pathogenicity is the ability of a microbe to cause disease, while virulence describes the degree or severity of disease caused. Virulence factors are molecules or structures that enable pathogens to overcome host defenses and cause damage.

• Virulence Factors Include:

  • Adhesins (e.g., fimbriae, pili, capsules)

  • Toxins (exotoxins and endotoxins)

  • Enzymes (e.g., coagulase, kinases, proteases, lipases)

  • Iron-binding proteins (e.g., siderophores)

  • Immune evasion mechanisms (e.g., capsules, antigenic variation)

• Attenuation: Pathogens may lose virulence factors when grown in cell culture, becoming less able to cause disease (attenuated), but may still be infectious. Attenuated strains are sometimes used in vaccines.

Establishing Infection: The Five Steps

To cause disease, a pathogen must complete five general tasks:

1. Enter the host via a suitable portal of entry

2. Adhere to host tissues using adhesins

3. Invade tissues and obtain nutrients (using invasins and enzymes)

4. Replicate while evading immune defenses

5. Transmit to a new host via a portal of exit

Portals of Entry and Exit

Pathogens use specific routes to enter and exit the host, often the same for both processes.

Adhesion and Biofilms

• Adhesins: Molecules (often on fimbriae, pili, or capsules) that allow pathogens to attach to host cell receptors.

• Biofilms: Complex communities of microbes attached to surfaces (e.g., catheters, prosthetics); 60-80% of human infections are associated with biofilms.

Invasion and Nutrient Acquisition

• Invasins: Enzymes that help pathogens invade tissues by breaking down host barriers or inducing host cell uptake.

• Iron Acquisition: Pathogens secrete siderophores to scavenge iron from host proteins (e.g., transferrin).

• Extracellular Enzymes:

  • Lipases: Break down lipids

  • Proteases: Break down proteins

  • Coagulase: Promotes clot formation to evade immune response

  • Kinases: Modify host signaling or dissolve clots

Toxins: Exotoxins and Endotoxins

Toxins are molecules that cause adverse effects in the host, contributing to disease symptoms.

• Exotoxin Families:

  • Type I: Membrane-acting, bind to receptors, alter gene expression

  • Type II: Membrane-damaging, form pores or disrupt membranes, cause cell lysis

  • Type III: Intracellular (AB toxins), B domain binds, A domain is active inside cell

• Superantigens: Exotoxins that overstimulate the immune system, causing severe inflammation and potentially shock.

• Toxemia: Presence of toxins in the bloodstream.

• LD50 and ID50:

  • : Number of organisms needed to infect 50% of hosts

  • : Dose of toxin/pathogen that kills 50% of hosts

Immune Evasion Strategies

• Capsules: Prevent phagocytosis by masking bacterial surface antigens.

• Antigenic Variation: Pathogens alter surface molecules to evade immune detection.

• Antigenic Mimicry: Pathogens produce molecules resembling host components.

• Antigenic Masking: Pathogens coat themselves with host molecules.

• Latency: Pathogens remain dormant within host cells, reactivating later (e.g., herpesviruses, HIV).

• Intracellular Lifestyle: Some pathogens (e.g., Listeria monocytogenes, Mycobacterium tuberculosis) survive and replicate inside host cells.

• Immune Suppression: Pathogens may directly target immune cells, degrade antibodies, or interfere with immune signaling.

Transmission and Reservoirs

• Portals of Exit: Routes include feces, urine, blood, respiratory secretions, and other bodily fluids.

• Reservoirs: Environmental sources (soil, water), animals, humans, or fomites (inanimate objects).

• Fomites: Non-living objects that can transmit pathogens (e.g., doorknobs, medical equipment).

Biosafety Levels and Infection Control

Laboratories and healthcare settings use biosafety levels (BSL) and precautions to prevent the spread of infectious agents.

• Standard Precautions: Universal measures (hand hygiene, PPE, safe injection, cleaning) for all patients.

• Transmission-Based Precautions: Additional measures for known/suspected infections (contact, droplet, airborne).

• Contact Precautions: Gowns, gloves, dedicated equipment (e.g., for MRSA, C. difficile).

• Droplet Precautions: Masks, limited transport (e.g., for influenza, rubella).

• Airborne Precautions: Specialized ventilation, respirators (not detailed in original notes, but relevant for BSL-3/4 agents).

Key Terms and Quick Reference

Summary Table: Exotoxins vs. Endotoxins

Formulas

• Infectious Dose 50 (ID50):

• Lethal Dose 50 (LD50):

Additional info:

• Some details about airborne precautions and specific examples of BSL-3/4 agents were inferred for completeness.

• Tables were reconstructed and expanded for clarity and study utility.