USMLE

Haemophilus influenzae

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Bacteria - Gram Negative
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Haemophilus influenzae 

  • Note: contrast vs. haemophilus ducreyi (produces genital ulcers)
  • Many different strains of H. influenzae exist
    • Serotype type B (Hib) is the most pathogenic
  • Characteristics
    • Gram negative
    • Coccobacilli or bacilli
    • Facultative Anaerobe 
    • Oxidase + 
    • Non-motile 
    • Requires factors V (NAD+) and X (hematin) for growth on chocolate agar
      • Can also be grown with S aureus, which provides factor V via RBC hemolysis
  • Virulence factors
    • Produces IgA protease 
    • Polysaccharide capsule 
      • capsule composed of polyribosylribitol phosphate (PRP)
      • Asplenic patients (past surgery, sickle cell), people with C1 complement deficiency, or people with B-cell immunodeficiencies (e.g. Bruton’s agammaglobulinemia) are particularly susceptible to severe infections by encapsulated organisms
  • Presentation
    • Important: H. influenzae does not cause the flu (Influenza virus causes the flu)
    • Aerosol transmission
    • Respiratory/airway infections
      • Otitis media/sinusitis
      • Pneumonia/bronchitis
      • Caused by non-serotypable H. influenzae
      • Conjunctivitis
    • Meningitis 
      • Caused by H. influenzae type B (Hib)
      • Hib may also cause epiglottitis
    • Can produce culture-negative endocarditis
      • Part of the HACEK organisms (Haemophilus, Actinobacillus, Cardiobacter, Eikenella, Kingella)
  • Treatment
    • Amoxicillin (with clavulanate)
      • First-line for mucosal infections (sinusitis, otitis media)
    • Ceftriaxone
      • First-line for meningitis
    • Rifampin prophylaxis for close contacts 
  • Prophylaxis
    • Haemophilus influenza type b (Hib) vaccine
      • contains bacterial capsular polysaccharide conjugated with diphtheria toxoid, which improves immunogenicity
      • given between 2 and 18 months of age