Normal Microbiota and Pathogenicity

Normal Microbiota - Our Tiny Tenants

• Definition: Resident microbes (commensals) on/in healthy individuals; vital for health.
• Key Sites & Predominant Flora:
  • Skin: Staphylococcus epidermidis, Propionibacterium acnes.
  • Nasopharynx: S. aureus (carrier), Viridans streptococci.
  • Mouth: S. mutans (dental caries), anaerobes (Bacteroides).
  • Colon: Highest load ($10^{11}-10^{12}$/g); >95% anaerobes (Bacteroides fragilis, Clostridium); E. coli.
  • Vagina: Lactobacillus spp. (maintains acidic pH, protective).
• Beneficial Roles:
  • Vitamin K & B synthesis.
  • Colonization resistance (vs pathogens).
  • Aids immune development.
• Sterile Sites (Normally): Blood, CSF, Lungs (LRT), Bladder, internal organs.
• Dysbiosis & Pathogenicity:
  • Dysbiosis: Imbalance in flora; can cause disease.
  • Opportunistic infections: Flora pathogenic if host immunocompromised or flora displaced.

⭐ Bacteroides fragilis: most common colon anaerobe; causes intra-abdominal infections if displaced.

Pathogenicity & Virulence - Villains' Toolkit

• Pathogenicity: Ability to cause disease.

• Virulence: Degree of pathogenicity. $LD_{50}$ (Lethal Dose): ↓ = ↑ virulence.

• Infectivity: Ability to colonize. $ID_{50}$ (Infectious Dose): ↓ = ↑ infectivity.

• Virulence Factors (Arsenal):

  • Adhesion: Critical for colonization.
    • Pili, adhesins (M protein - S. pyogenes, Opa protein).
  • Invasion & Spread: Aid tissue penetration.
    • Enzymes: Hyaluronidase, collagenase, coagulase, kinases, DNase.
  • Evasion of Host Defenses:
    • Capsules (S. pneumoniae): Antiphagocytic, mask PAMPs.
    • Biofilms: Resist antibiotics/phagocytosis.
    • Antigenic variation (Influenza).
    • IgA proteases.
  • Toxins:
    • Exotoxins: Secreted proteins (G+/G-). Potent, specific. Mostly heat-labile. Toxoids (vaccines). Plasmid/phage coded.
      • Types: A-B (Diphtheria, Cholera), membrane-disrupting (lecithinase), superantigens (TSST-1).
    • Endotoxins: Lipid A (LPS of G-). Released on lysis. Fever, DIC, shock (TNF-α, IL-1). Heat-stable. No toxoids.

    ⭐ Lipid A of LPS is the endotoxin component of Gram-negative bacteria, responsible for septic shock.

Host-Pathogen Tango - Infection Unfolded

Dynamic interplay: invading microbes vs. host defenses, shaping infection's course and outcome.

• Infection Unfolds:
  • Entry: Via portals (e.g., respiratory, GI). Adherence via adhesins.
  • Invasion & Spread: Localized or systemic dissemination.
  • Multiplication: Colonization, often evading host immunity (e.g., capsules, biofilms).
  • Damage: Caused by pathogen (toxins, enzymes) or host response (immunopathology).
• Critical Factors:
  • Pathogen: Virulence factors, inoculum size (ID₅₀).
  • Host: Immune status, genetic susceptibility, age.
• Possible Outcomes:
  • Resolution, asymptomatic carrier, latency, chronic disease, or death.

⭐ Infectious Dose 50 (ID₅₀) is the number of organisms needed to infect 50% of exposed individuals. A lower ID₅₀ indicates greater virulence.

Opportunistic Infections - Friends Turned Foes

• Commensals or low-virulence microbes causing disease when host defenses are weakened.
• Key predisposing factors:
  • Immunodeficiency (e.g., HIV, chemotherapy, steroids).
  • Disruption of normal flora (e.g., broad-spectrum antibiotics → C. difficile).
  • Breached anatomical barriers (e.g., trauma, surgery, catheters, burns).
• Common examples:
  • Candida albicans (oral thrush, vulvovaginitis).
  • Staphylococcus epidermidis (catheter-related infections, endocarditis).
  • Pseudomonas aeruginosa (burn wounds, ventilator-associated pneumonia).
  • Pneumocystis jirovecii (PCP pneumonia in AIDS patients).

⭐ Pseudomonas aeruginosa is notorious for causing infections in burn patients and individuals with cystic fibrosis.

• 📌 Remember SPACE organisms for nosocomial infections (often opportunistic): Serratia, Pseudomonas, Acinetobacter, Citrobacter, Enterobacter.

High‑Yield Points - ⚡ Biggest Takeaways

• Normal microbiota offers colonization resistance, preventing pathogen invasion.
• Opportunistic pathogens primarily infect immunocompromised individuals or disrupted microbiota sites.
• Pathogenicity is the capacity to cause disease; virulence quantifies this capacity.
• Adherence via structures like pili and adhesins is critical for infection initiation.
• Exotoxins (secreted proteins) and endotoxins (LPS of Gram-negatives) are key virulence factors.
• Biofilms contribute to chronic infections and increased antimicrobial resistance.
• Koch's postulates are criteria to link a specific microbe to a specific disease.