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Microbiology

Organism Pathogenicity

Understanding the pathogenicity of lower respiratory tract pathogens is crucial for effective diagnosis, treatment, and prevention of respiratory infections. Pathogenicity is defined as the ability of a microorganism to cause disease, and it is determined by the pathogen’s capacity to:

  1. Colonize: the host
  2. Invade: tissues
  3. Evade: the host’s immune defenses
  4. Produce: toxins or other harmful substances

This is especially important when the source of an infection is in the lower respiratory tract

Organism Pathogenicity in Lower Respiratory Infections

Streptococcus pneumoniae

  • Etiology: Gram-positive, lancet-shaped diplococci
  • Transmission: Respiratory droplets
  • Virulence Mechanisms
    • Capsule: Prevents phagocytosis (key virulence factor)
    • Adhesins: Facilitate attachment
    • Pneumolysin: Damages host cells
    • IgA Protease: Destroys host antibodies
  • Pathogenicity
    • Pneumonia, sometimes severe
    • Inflammation and tissue damage
    • May lead to sepsis

Haemophilus influenzae

  • Etiology: Gram-negative coccobacilli or pleomorphic rods
  • Transmission: Respiratory droplets
  • Virulence Mechanisms
    • Capsule: Antiphagocytic (Type b is most virulent)
    • Adhesins: Promote adherence
    • Endotoxin (LPS): Triggers inflammation
  • Pathogenicity
    • Pneumonia, exacerbations of COPD
    • Inflammation and tissue damage. Potential for complications

Moraxella catarrhalis

  • Etiology: Gram-negative diplococci
  • Transmission: Respiratory droplets
  • Virulence Mechanisms
    • Adhesins: Facilitate attachment
    • Outer membrane proteins: Involved in adherence and immune evasion
    • Beta-Lactamase: Many strains produce it. This is an enzyme
  • Pathogenicity
    • Bronchitis and pneumonia
    • Inflammation, associated with airway obstruction

Legionella pneumophila

  • Etiology: Gram-negative rod
  • Transmission: Aerosols from environmental sources (e.g., water systems, cooling towers)
  • Virulence Mechanisms
    • Intracellular Survival: Survives and replicates within alveolar macrophages
    • Effector Proteins: Manipulate host cell processes
    • Type IV secretion system: Transfers effector proteins
  • Pathogenicity
    • Legionnaires’ disease (severe pneumonia)
    • Pontiac fever (a milder illness)
    • Pneumonia. Inflammation. Organ damage
  • Pathogenesis: Inhalation of aerosols containing the bacteria, followed by the replication within alveolar macrophages

Staphylococcus aureus

  • Etiology: Gram-positive cocci in clusters
  • Transmission: Direct contact, respiratory droplets
  • Virulence Mechanisms
    • Adherence: Surface proteins
    • Enzymes: Coagulase, hyaluronidase
    • Immune Evasion: Protein A
    • Toxins: Cytotoxins, superantigens
  • Pathogenicity
    • Pneumonia, often severe
    • Inflammation, tissue damage, sepsis
    • Can cause empyema, lung abscesses
  • Pathogenesis: Aspiration, hematogenous spread, or direct spread from other infections

Pseudomonas aeruginosa

  • Etiology: Gram-negative rod
  • Transmission: Environmental (water, soil, contaminated surfaces), healthcare settings
  • Virulence Mechanisms
    • Adhesins: Promote attachment
    • Capsule: Prevents phagocytosis
    • Exotoxins: Exotoxin A
    • Enzymes: Proteases, elastases
    • Biofilm Formation: Forms biofilms on medical devices
    • Antibiotic Resistance: Significant resistance
  • Pathogenicity
    • Pneumonia, often in immunocompromised patients
    • Inflammation, tissue damage, sepsis
    • Frequently multidrug-resistant. Chronic infection
  • Pathogenesis: Aspiration, colonization, direct spread from other infections

Acinetobacter baumannii Complex

  • Etiology: Gram-negative coccobacilli
  • Transmission: Healthcare settings (contaminated equipment, hands)
  • Virulence Mechanisms
    • Adhesins: Promote attachment
    • Biofilm Formation: Forms biofilms
    • Antibiotic Resistance: Significant resistance
  • Pathogenicity
    • Pneumonia, often ventilator-associated (VAP)
    • Inflammation, tissue damage, sepsis
    • Difficult to treat, highly multidrug-resistant
  • Pathogenesis: Aspiration and spread from contaminated sources

Stenotrophomonas maltophilia

  • Etiology: Gram-negative rod
  • Transmission: Healthcare settings, environment
  • Virulence Mechanisms
    • Biofilm Formation: Biofilms protect the organism from immune defenses and antibiotics
    • Enzymes: Proteases, lipases
    • Antibiotic Resistance: Often inherently resistant
  • Pathogenicity
    • Pneumonia, ventilator-associated pneumonia
    • Inflammation, tissue damage
  • Pathogenesis: Aspiration, colonization, and direct spread

Burkholderia cepacia Complex (Bcc)

  • Etiology: Gram-negative rods
  • Transmission: Environment, person-to-person (in CF patients)
  • Virulence Mechanisms
    • Adhesins: Promote attachment
    • Biofilm Formation: Forms biofilms
    • Enzymes: Proteases
  • Pathogenicity
    • Pneumonia, especially in cystic fibrosis (CF)
    • Pulmonary decline
    • Can be transmitted between CF patients
  • Pathogenesis: Inhalation or direct spread

Implications of Pathogenicity

  • Clinical Presentation: Symptoms and severity of disease vary based on the specific pathogen and its virulence mechanisms
  • Diagnosis: Knowledge of virulence factors is crucial for guiding the choice of appropriate diagnostic tests (e.g., culture, antigen detection, PCR)
  • Treatment: The specific virulence factors and the antibiotic resistance profiles help determine the correct treatment
  • Prevention: Understanding transmission routes and virulence factors informs the development of infection control measures (e.g., hand hygiene, respiratory etiquette)
  • Chronic Infections and CF
    • Biofilm formation, which can protect the bacteria, and cause chronic infections
    • Immune evasion

Key Terms

  • Pathogenicity: The ability of a microorganism to cause disease
  • Virulence: The degree of pathogenicity
  • Etiology: The cause of a disease
  • Transmission: The spread of a pathogen
  • Adhesins: Surface structures or proteins that attach to host cells
  • Capsule: A protective layer that surrounds bacteria
  • Endotoxin (LPS): A component of the outer membrane of Gram-negative bacteria that triggers inflammation
  • Exotoxins: Toxins secreted by bacteria
  • Biofilm: A community of microorganisms that adhere to a surface
  • Sepsis: A life-threatening response to infection
  • Antibiotic Resistance: Ability of bacteria to survive antibiotics
  • Community-Acquired Pneumonia (CAP): Pneumonia that is acquired outside of a healthcare setting
  • Healthcare-Associated Pneumonia (HCAP): Pneumonia acquired in a healthcare setting
  • Ventilator-Associated Pneumonia (VAP): Pneumonia that occurs in patients on mechanical ventilation
  • Cystic Fibrosis (CF): A genetic disorder affecting the lungs, digestive system, and other organs
  • Empyema: Pus in the pleural space
  • Pleurisy: Inflammation of the pleura
  • Exacerbation: Worsening of a disease
  • Aerosols: Tiny particles that are suspended in the air