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TAXONOMY: Bacteria -> Proteobacteria -> Gammaproteobacteria -> Pasteurellales -> Pasteurellaceae -> Haemophilus -> Haemophilus influenzae
Haemophilus influenzae (formerly called Pfeiffer's bacillus or Bacillus influenzae) is a Gram-negative, coccobacillary, facultatively anaerobic pathogenic bacterium belonging to the Pasteurellaceae family. H. influenzae was first described in 1892 by Richard Pfeiffer during an influenza pandemic. The bacterium was mistakenly considered to be the cause of influenza until 1933 when the viral cause of influenza became apparent, and is still colloquially known as 'bacterial influenza'. H. influenzae is responsible for a wide range of localized and invasive infections. This species was the first free-living organism to have its entire genome sequenced. In 1930, two major categories of H. influenzae were defined: the unencapsulated strains and the encapsulated strains. Encapsulated strains were classified on the basis of their distinct capsular antigens. There are six generally recognized types of encapsulated H. influenzae: a, b, c, d, e, and f. Genetic diversity among unencapsulated strains is greater than within the encapsulated group. Unencapsulated strains are termed nontypable (NTHi) because they lack capsular serotypes; however, they can be classified by multilocus sequence typing. The pathogenesis of H. influenzae infections is not completely understood, although the presence of the capsule in encapsulated type b (Hib), a serotype causing conditions such as epiglottitis, is known to be a major factor in virulence. Their capsule allows them to resist phagocytosis and complement-mediated lysis in the nonimmune host. The unencapsulated strains are almost always less invasive; they can, however, produce an inflammatory response in humans, which can lead to many symptoms. Vaccination with Hib conjugate vaccine is effective in preventing Hib infection, but does not prevent infection with NTHi strains. Identified as a constituent of the oral microbiome by Human Oral Microbiome Database.
This species has been identified as a resident in the human gastrointestinal tract based on the phylogenetic framework of its small subunit ribosomal RNA gene sequences.[PMC 4262072]
|COGEM released a comprehensive database of pathogenicity assessment of around 2575 bacterial species in 2011. The database ranks the pathogenicity of species on a scale of 1 to 4. Haemophilus influenzae ranks 2 on this scale: Species that can cause diseases in humans or animals, which are
unlikely to spread in the human population and for which an adequate
prophylaxis or therapy exists|