Haemophilus species

MICROBIOLOGY

• Small aerobic Gram-negative coccobacillus found mainly in the respiratory tract.
  • Fastidious organism, factors X (hemin, haemophilius from Latin "blood loving") and V (nicotinamide-adenine-dinucleotide) required for growth on chocolate agar, the preferred media
• Haemophilus influenzae: two categories causing human colonization or infection
  
  • Typeable strains: six types generally recognized (Types a-f).
    • Encapsulated, type B strain (Hib, with capsular antiphagocytosis and anticomplement virulence factors) historically accounts for most invasive infections and bacteremic pneumonia. Now rarely encountered due to childhood Hib vaccine success.
    • H. influenzae serotype a (Hia) increasingly noted as cause of severe, invasive infection. Emergence due to intrinsic capsular virulence factors of Hia was well as pressure from success of Hib immunization[5].
      • Predominantly noted in U.S. and Canada. Incidence in Europe, low.
      • In one series, accounted for ~30% of invasive H. influenzae infection in Manitoba.
    • Type f strain also increasingly recognized as cause of severe infection, though less common than Hia.
  • Non-typeable strains: less invasive and cause more otitis media, epiglottitis, acute exacerbations of chronic bronchitis (AECB), sinusitis and nonbacteremic pneumonia.
    • Increasingly recognized as cause of URTIs, pneumonia and occasional bacteremia in all age groups.
    • High colonization rates in children, up to 70%.

• Other species: all are part of normal flora of human oral and pharyngeal cavities but may also be causes of bacterial respiratory tract infections and endocarditis, similar to H. influenzae.
  
  • H. parainfluenzae (most common)
  • H. aphrophilus
  • H. paraphrophilus
  • H. parahaemolyticus
  • H. segnis a
  • Note: H. aphrophilus and H. segnis are now classified to new genus: Aggregatibacter aphrophilus and Aggregatibacter segnis[18].
• H. ducreyi, agent of Chancroid, see separate module.