Haemophilus influenzae

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Haemophilus influenzae, formerly called Pfeiffer's bacillus or Bacillus influenzae, is a non-motile Gram-negative coccobacillus first described in 1892 by Richard Pfeiffer during an influenza pandemic. It is generally aerobic, but can grow as a facultative anaerobe. H. influenzae was mistakenly considered to be the cause of the common flu until 1933, when the viral etiology of the flu became apparent. Still, H. influenzae is responsible for a wide range of clinical diseases.

H. influenzae was the first free-living organism to have its entire genome sequenced. Haemophilus was chosen because one of the project leaders, Nobel laureate Hamilton Smith, had been working on it for decades and was able to provide high-quality DNA libraries. The genome consists of 1,830,140 base pairs of DNA in a single circular chromosome that contains 1740 protein-coding genes, 58 transfer RNA genes tRNA, and 18 other RNA genes. The sequencing method used was Whole genome shotgun. The sequencing project, completed and published in Science in 1995, was conducted at The Institute for Genomic Research.[1]




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, such as epiglottitis. Vaccination with Hib conjugate vaccines is effective in preventing infection, and several vaccines are now available for routine use.



Most strains of H. influenzae are opportunistic pathogens - that is, they usually live in their host without causing disease, but cause problems only when other factors (such as a viral infection or reduced immune function) create an opportunity. There are six generally recognized types of H. influenzae: a, b, c, d, e, and f.[2]

Naturally-acquired disease caused by H. influenzae seems to occur in humans only. In infants and young children, H. influenzae type b (Hib) causes bacteremia, and acute bacterial meningitis. Occasionally, it causes cellulitis, osteomyelitis, epiglottitis, and joint infections. Due to routine use of the Hib conjugate vaccine in the U.S. since 1990, the incidence of invasive Hib disease has decreased to 1.3/100,000 in children. However, Hib remains a major cause of lower respiratory tract infections in infants and children in developing countries where vaccine is not widely used. Unencapsulated H. influenzae (non-B type) causes ear (otitis media) and eye (conjunctivitis) infections and sinusitis in children, and is associated with pneumonia.



A specimen with H. influenzae is Gram-stained to show G - ve, coccobacilli, with no specific arrangement. Then, it's grown on nutrient agar plate with added X & V factors. Also, growth on blood agar would show the satellitism phenomenon, which is the growth of H. influenzae around a streak of S. aureus line of growth. On Chocolate agar, the organism grows readily as this medium is nothing but a heated blood agar, which means that RBC's are lysed upon heating, releasing factor V for the organism to grow (X is already in blood). It shows small, convex, smooth, pale, grey or transparent colonies. The organism is also catalase and oxidase positive. Serological testing demonstrates the capsular polysaccharide, which is usually of the b type as it's the most common to cause infections.

It's worth mentioning that in case of epiglottitis, a blood culture should be taken which is then subcultured on blood agar. Further identification is also performed as mentioned above.


Interaction with Streptococcus pneumoniae

Both H. influenzae and S. pneumoniae can be found in the upper respiratory system of humans. A study of competition in a laboratory revealed that, in a petri dish, S. pneumoniae always overpowered H. influenzae by attacking it with hydrogen peroxide and stripping off the surface molecules that H. influenzae needs for survival.

When both bacteria are placed together into a nasal cavity, within 2 weeks, only H. influenzae survives. When both are placed separately into a nasal cavity, each one survives. Upon examining the upper respiratory tissue from mice exposed to both bacteria species, an extraordinarily large number of neutrophils (immune cells) was found. In mice exposed to only one bacteria, the cells were not present.

Lab tests showed that neutrophils exposed to dead H. influenzae were more aggressive in attacking S. pneumoniae than unexposed neutrophils. Exposure to dead H. influenzae had no effect on live H. influenzae.

Two scenarios may be responsible for this response:

  1. When H. influenzae is attacked by S. pneumoniae, it signals the immune system to attack the S. pneumoniae
  2. The combination of the two species together triggers an immune system response that is not set off by either species individually.

It is unclear why H. influenzae is not affected by the immune response.[3]


See also

  • Maurice Hilleman
  • Hattie Alexander
  • Hib vaccine



  1. ^ Fleischmann R, Adams M, White O, Clayton R, Kirkness E, Kerlavage A, Bult C, Tomb J, Dougherty B, Merrick J (1995). "Whole-genome random sequencing and assembly of Haemophilus influenzae Rd". Science 269 (5223): 496-512. PMID 7542800. 
  2. ^ Ryan KJ; Ray CG (editors) (2004). Sherris Medical Microbiology, 4th ed., McGraw Hill, pp. 396–401. ISBN 0838585299. 
  3. ^ Lysenko E, Ratner A, Nelson A, Weiser J (2005). "The role of innate immune responses in the outcome of interspecies competition for colonization of mucosal surfaces". PLoS Pathog 1 (1): e1. PMID 16201010. 


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