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Haemophilus influenzae

Cell Wall: Gram Negative Shape: Coccoid Rod
Metabolism: Facultative Anaerobe
Culture and Detection
  • Selective Media for H. influenzae is a chocolate agar supplemented with Factors V (NAD) and Factor X (hemin).
Virulence Factors
  • Capsule
    • Six serotypes of H. influenzae possess an antiphagocytic polysaccharide capsule and are identified by letters a through f. These strains are highly invasive and can disseminate systemically with the Type b strain being the most dangerous. Several strains lack capsule and are termed "Non-typable". These are only locally invasive and can only disseminate to structures contiguous with the nasopharynx.
  • H. influenzae is transmitted by aerosolized droplets. Non-typable trains are part of the normal nasopharyngeal flora in many individuals.
Clinical Consequences
  • Typable Strains (Especially Type b)
    • Typable strains of H. influenzae (especially Type b) invade through the nasopharyngeal mucosa, gain access to the blood stream, and can spread to distant sites
    • Meningitis: Usually affects infants less than 2yo
    • Acute Epiglottitis: Usually affects children less than 6yo and results in a characteristic "Cherry red" epiglottis which can be life-threatening due to potential airway obstruction]
    • Septic Arthritis: Usually occurs only in children <6yo
    • Community-acquired Pneumonia: Usually occurs in infants
  • Non-typable Strains
    • Non-typable strains of H. influenzae are only locally invasive and generally cause disease only in adults with reduced immunity or superficial ear infections in the young
    • Community-acquired Pneumonia: Non-typable strains are a frequent cause of Pneumonia especially in those with COPD
    • Otitis Media: H. influnzae is the most common cause of Otitis Media in infants
  • Protective immunity to Typable strains of H. influenzae is through humoral immunity which develops antibodies to the polysaccharide capsule. The predilection of Type b infections in infants between 6mo - 2yo is due to an antibody window.
  • A conjugate vaccine is available for the Type b serotype and is frequently administered to infants. It consists of the Type b capsular polysaccharide fused to diphtheria toxin.
  • First Choices
    • Typable strains: Third Generation Cephalosporins such as cefotaxime and ceftriaxone
    • Non-typable strains: Amoxicillin
  • Additional Choices
    • Ciprofloxacin or Bactrim