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Microbiology

Organism Pathogenicity

Understanding the pathogenicity of upper respiratory tract (URT) pathogens is crucial for effective diagnosis, treatment, and prevention of common infections. Pathogenicity is the ability of a microorganism to cause disease, depending on the microorganism’s ability to:

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

Organism Pathogenicity in Upper Respiratory Infections

Streptococcus pyogenes (Group A Streptococcus, GAS)

  • Etiology: Gram-positive cocci in chains
  • Transmission: Respiratory droplets
  • Virulence Mechanisms
    • M Protein: Major virulence factor; inhibits phagocytosis
    • Capsule: Hyaluronic acid capsule; camouflage
    • Exotoxins: Streptococcal pyrogenic toxins (SPEs); superantigens that can cause toxic shock syndrome
    • Enzymes: Streptolysin S; damages host cells
  • Pathogenicity
    • Pharyngitis (“strep throat”)
    • Scarlet fever
    • Skin infections
    • Rarely, rheumatic fever, acute glomerulonephritis (post-streptococcal sequelae)
  • Pathogenesis: Adherence to the pharyngeal mucosa, followed by local inflammation and the production of toxins

Corynebacterium diphtheriae

  • Etiology: Gram-positive rod
  • Transmission: Respiratory droplets
  • Virulence Mechanisms
    • Diphtheria Toxin: A potent exotoxin that inhibits protein synthesis; encoded by a prophage
  • Pathogenicity
    • Diphtheria (characterized by a pseudomembrane in the pharynx)
    • Can cause systemic effects
  • Pathogenesis: Colonization of the URT. The toxin is released. The toxin causes cell death and the formation of a pseudomembrane

Bordetella pertussis

  • Etiology: Gram-negative coccobacillus
  • Transmission: Respiratory droplets
  • Virulence Mechanisms
    • Pertussis Toxin (PT): Major virulence factor; interferes with host cell signaling
    • Adhesins: Filamentous hemagglutinin (FHA); attaches to host cells
    • Tracheal Cytotoxin: Damages ciliated respiratory epithelial cells
    • Other toxins
  • Pathogenicity
    • Whooping cough (pertussis)
    • Characteristic stages: catarrhal, paroxysmal, convalescent
  • Pathogenesis: Attachment to respiratory epithelial cells, leading to inflammation and toxin production

Neisseria gonorrhoeae

  • Etiology: Gram-negative diplococci
  • Transmission: Sexual contact
  • Virulence Mechanisms
    • Adhesins: Attach to host cells
    • Antigenic Variation: Changes surface antigens to evade the immune system
    • Endotoxin (LPS): Triggers inflammation
  • Pathogenicity
    • Pharyngitis (less common in the URT)
  • Pathogenesis: Direct contact of the organism with the mucosal surfaces, often as a result of sexual activity

Haemophilus influenzae (non-typeable)

  • Etiology: Gram-negative coccobacilli
  • Transmission: Respiratory droplets
  • Virulence Mechanisms
    • Adhesins: Promote adherence
    • Endotoxin (LPS): Triggers inflammation
  • Pathogenicity
    • Otitis media, sinusitis, and conjunctivitis
    • Can cause pneumonia and other invasive infections
  • Pathogenesis: Adherence to the respiratory epithelium and inflammation

Streptococcus pneumoniae

  • Etiology: Gram-positive, lancet-shaped diplococci
  • Transmission: Respiratory droplets
  • Virulence Mechanisms
    • Capsule: Prevents phagocytosis
    • Adhesins: Promote attachment
    • Pneumolysin: Damages host cells
    • IgA Protease: Degrades host antibodies
  • Pathogenicity
    • Pneumonia, otitis media, and sinusitis
    • Inflammation, tissue damage, and sepsis
  • Pathogenesis: Colonization, entry, and spread

Moraxella catarrhalis

  • Etiology: Gram-negative diplococci
  • Transmission: Respiratory droplets
  • Virulence Mechanisms
    • Adhesins: Promotes adherence
    • Outer membrane proteins: Adherence and immune evasion
    • Beta-Lactamase: Produces this to resist antibiotics
  • Pathogenicity
    • Otitis media and sinusitis
  • Pathogenesis: Adherence to the respiratory epithelium and inflammation

Implications of Pathogenicity

  • Clinical Presentation: Determines the signs and symptoms (e.g., sore throat, cough, ear pain, nasal congestion)
  • Diagnosis: Guides the selection of appropriate diagnostic tests (e.g., culture, rapid antigen tests, molecular methods)
  • Treatment: Determines appropriate antimicrobial therapy or other interventions. For instance, only C. diphtheriae needs an antitoxin, while others need antibiotics
  • Prevention: Knowledge of transmission routes and virulence factors informs the development of preventive measures (e.g., vaccination, respiratory hygiene)

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 facilitate attachment
  • Capsule: A protective layer that surrounds bacteria
  • Toxins: Harmful substances produced by pathogens
  • Streptococcus pyogenes (Group A Streptococcus (GAS)): The main cause of strep throat
  • Corynebacterium diphtheriae: The causative agent of diphtheria
  • Diphtheria Toxin: The toxin that causes diphtheria
  • Bordetella pertussis: The causative agent of whooping cough
  • Pertussis Toxin: The toxin that causes whooping cough
  • Ciliated Respiratory Epithelial Cells: Cells lining the respiratory tract
  • Neisseria gonorrhoeae: Gram-negative bacteria that can infect the throat
  • Haemophilus influenzae: (non-typeable): Gram-negative coccobacilli
  • Streptococcus pneumoniae: Alpha-hemolytic bacteria
  • Moraxella catarrhalis: Gram-negative bacteria
  • Superantigen: A type of toxin