Haemophilus influenzae, also called Pfeiffer's bacillus or Bacillus influenzae, is a small Gram-negative, nonmotile, coccobacillary, facultatively anaerobic pathogenic bacterium that belongs to the family Pasteurellaceae, genus Haemophilus. Encapsulated strains of Haemophilus influenzae are small coccobacilli with a size of 0.2-0.8 um. They are classified based on their distinct capsular antigens. Six types of encapsulated Haemophilus influenzae have been generally identified and are named as type a, b, c, d, e and f. Unencapsulated strains, however, have a greater genetic diversity than the encapsulated strains. Because of the lacking of capsular serotypes, unencapsulated strains are termed nontypable (NTHi). But they can also be classified through multilocus sequence typing.
Fig. 1 Haemophilus influenzae type b (Hib)
Haemophilus influenzae has been mistakenly recognized as the cause of influenza for a quite long time before the right viral cause of influenza was identified in 1933. It is the cause of a wide range of localized and invasive infections. Haemophilus influenzae highly adapted to its human hosts and is widespread among the human population. It exists in the nasopharynx of about 75% of healthy children and adults. In some cases it can be found in the oral cavity. No other species was found to be a host of Haemophilus influenzae.
Most strains of this bacterium are opportunistic pathogens. This means that they usually live in the hosts without any problems, but can cause diseases only when other factors create some opportunities, such as reduced immune function, viral infection and chronically inflamed tissues. Haemophilus influenzae type b (Hib) causes bacteremia, epiglottitis, pneumonia and acute bacterial meningitis in infants and children. Sometimes it can also cause osteomyelitis, cellulitis, and infectious arthritis. Hib is a major cause of lower respiratory tract infections in infants and young children in developing areas, where there is a lack of vaccines. Unencapsulated strains of Haemophilus influenzae can causes eye infections (conjunctivitis), ear infections (otitis media), and sinusitis in children. They are also found to be associated with pneumonia.