Haemophilus influenzae, formerly called Pfeiffer's bacillus, is a non-motile Gram-negative coccobacillus first described in 1892 by Dr. Robert Pfeiffer during the influenza pandemic. It is generally aerobic, but can grow as a facultative anaerobe. Haemophilus influenzae was mistakenly considered to be the cause of the common flu until 1933, when the viral etiology of the flu became apparent. Still, Haemophilus influenzae is responsible for a wide range of clinical diseases. Because of its small genome, Haemophilus influenzae became the first free-living organism with its entire genome sequenced.

Serotypes

In 1930, 2 major categories of H. influenzae were defined: the unencapsulated strains and the encapsulated strains. The pathogenesis of H. influenzae infections is not completely understood, although the presence of the encapsulated type b (HiB) is known to be the major factor in virulence. Their capsule allows them to resist phagocytosis and complement-mediated lysis in the non-immune host. Unencapsulated strains are less invasive, but they are able to induce an inflammatory response that causes disease. Vaccination with Hib conjugate vaccines is effective in preventing infection, and several vaccines are now available for routine use.

Diseases

Naturally-acquired disease caused by H. influenzae seems to occur in humans only. In infants and young children, the Haemophilis influenzea type B causes bacteremia and acute bacterial meningitis. Occasionally, it causes obstructive laryngitis, cellulitis, osteomyelitis and joint infections. Unencapsulated H. influenzae causes ear infections and sinusitis in children and is associated with pneumonia.

Additional links:

HiB information on the World Health Organization (WHO) site.

Fact sheet on the CDC site.