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  • Genus: Haemophilus
  • Species: influenzae

  • Haemophilus influenzae is responsible for producing a variety of infections including meningitis and respiratory infections.
  • Six serological types (a,b,c,d,e,f) based on the antigenic structure of the capsular polysaccharides are recognized. Nonencapsulated strains are (by definition) nontypable.
  • Other species of Haemophilus include: H. parainfluenzae (pneumonia, endocarditis), H. ducreyi (venereal chancre) and H. aegypticus (conjunctivitis).

  • The genus Haemophilus is composed of Gram-negative coccobacilli.
  • These organisms are fastidious and require factors X (hemin) and/or V (NAD).
  • Haemophilus possess LPS in the cell wall but produce no apparent extracellular toxins.

  • The organisms colonize the nasopharynx and are spread by direct contact. Haemophilus are capable of penetrating the epithelium to produce a bacteremia that may lead to localization of the organisms in many organs. The capsule is Haemophilus' major virulence determinant yet unencapsulated strains produce ear, sinus and respiratory infections.
  • H. influenzae type b is the most common cause of bacterial meningitis in children aged 6 months-2 years. It is uncommon in adults because of protective antibody.
  • Cellulitis, conjunctivitis, epiglottitis and arthritis may also result from Haemophilus infection.
  • For pneumonia in adult men, unencapsulated H. influenzae is second only to the pneumococcus (S. pneumoniae). Those affected are usually chronic smokers, alcoholics or elderly.

  • Antibody directed against the polyribosyl-ribitol-phosphate (PRP) capsule is bactericidal.

  • Spread of Haemophilus is human to human. Day care centers are common sites for transmission from healthy, unaffected adults to susceptible infants.


  • Clinical: A Gram stain of cerebrospinal fluid may reveal the organisms. One can also detect capsular material directly.
  • Laboratory: The organisms are cultured on chocolate agar because it contains both factors X and V. Incubation in 10% CO2 is required.


  • Sanitary: Avoidance of carriers is not always possible.
  • Immunological: A vaccine against type b is available. Unfortunately, the vaccine is nonimmunogenic in infants where it is needed the most.
  • Chemotherapeutic: Third generation cephalosporins are probably the drugs of choice because of their ability to cross the blood-brain barrier and their bactericidal activity.

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