Haemophilus influenzae

RANK: Species

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]

Gut associated
Oral microbiome
Group 39
  • Streptococcus pneumoniae
  • Neisseria meningitidis
  • Legionella pneumophila
  • Staphylococcus aureus
  • Streptococcus pyogenes
  • Haemophilus influenzae
  • Streptococcus agalactiae
  • Corynebacterium jeikeium
  • Group 30
  • Mycoplasma genitalium
  • Mycoplasma gallisepticum
  • Aquifex aeolicus
  • Bartonella bacilliformis
  • Haemophilus influenzae
  • Neisseria gonorrhoeae
  • Synechocystis
  • Mycoplasma capricolum
  • Ureaplasma urealyticum
  • Mycoplasma penetrans
  • Chlamydia trachomatis
  • Mycoplasma pulmonis
  • Mycoplasma hyopneumoniae
  • Group 54
  • Neisseria meningitidis
  • Haemophilus ducreyi
  • Dichelobacter nodosus
  • Enterobacter
  • Pseudomonas aeruginosa
  • Haemophilus influenzae
  • Neisseria gonorrhoeae
  • Group 5
  • Staphylococcus epidermidis
  • Lactobacillus acidophilus
  • Bifidobacterium adolescentis
  • Chloroflexus aurantiacus
  • Neisseria gonorrhoeae
  • Saccharophagus degradans
  • Cytophaga hutchinsonii
  • Bacteroides fragilis
  • Clostridium perfringens
  • Enterobacter
  • Propionibacterium acnes
  • Gramella forsetii
  • Clostridium acetobutylicum
  • Staphylococcus aureus
  • Porphyromonas gingivalis
  • Bifidobacterium longum
  • Colwellia psychrerythraea
  • Pseudomonas aeruginosa
  • Bacteroides thetaiotaomicron
  • Haemophilus influenzae
  • Peptoclostridium difficile
  • Fusobacterium nucleatum