Haemophilus influenzae

Haemophilus influenzae, formerly called Pfeiffer's bacillus or Bacillus influenzae, Gram-negative, rod-shaped bacterium first described in 1892 by Richard Pfeiffer during an influenza pandemic. A member of the Pasteurellaceae family, it is generally aerobic, but can grow as a facultative anaerobe. H. influenzae was mistakenly considered to be the cause of influenza 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. The sequencing project was completed and published in 1995.

Serotypes
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. Several vaccines are now available for routine use against Hib, but vaccines are not yet available against NTHi.

Diseases
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.

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, pneumonia, and acute bacterial meningitis. On occasion, it causes cellulitis, osteomyelitis, epiglottitis, and infectious arthritis. 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 the vaccine is not widely used. Unencapsulated H. influenzae causes ear infections (otitis media), eye infections (conjunctivitis), and sinusitis in children, and is associated with pneumonia.

Diagnosis
Clinical diagnosis of H. influenzae is typically performed by bacterial culture or latex particle agglutinations. Diagnosis is considered confirmed when the organism is isolated from a sterile body site. In this respect, H. influenzae cultured from the nasopharyngeal cavity or sputum would not indicate H. influenzae disease, because these sites are colonized in disease-free individuals. However, H. influenzae isolated from cerebrospinal fluid or blood would indicate H. influenzae infection.

Culture
Bacterial culture of H. influenzae is performed on agar plates, the preferable one being chocolate agar, with added X(hemin) & V(NAD) factors at 37°C in a CO2-enriched  incubator. Blood agar growth is only achieved as a satellite phenomenon around other bacteria. Colonies of H. influenzae appear as convex, smooth, pale, grey or transparent colonies. Gram-stained and microscopic observation of a specimen of H. influenzae will show Gram-negative, coccobacilli, with no specific arrangement. The cultured organism can be further characterized using catalase and oxidase tests, both of which should be positive. Further serological testing is necessary to distinguish the capsular polysaccharide and differentiate between H. influenzae b and nonencapsulated species.

Although highly specific, bacterial culture of H. influenzae lacks in sensitivity. Use of antibiotics prior to sample collection greatly reduces the isolation rate by killing the bacteria before identification is possible. Beyond this, H. influenzae is a finicky bacterium to culture, and any modification of culture procedures can greatly reduce isolation rates. Poor quality of laboratories in developing countries has resulted in poor isolation rates of H. influenzae.

H. influenzae will grow in the hemolytic zone of Staphylococcus aureus on blood agar plates; the hemolysis of cells by S. aureus releases nutrients vital to its growth. H. influenzae will not grow outside the hemolytic zone of S. aureus due to the lack of nutrients in these areas. Fildes agar is best for isolation. In Levinthal medium capsulated strains show distinctive iridescence.

Latex particle agglutination
The latex particle agglutination test (LAT) is a more sensitive method to detect H. influenzae than culture. Because the method relies on antigen rather than viable bacteria, the results are not disrupted by prior antibiotic use. It also has the added benefit of being much quicker than culture methods. However, antibiotic sensitivity is not possible with LAT, so a parallel culture is necessary.

Molecular methods
Polymerase chain reaction (PCR) assays have been proven to be more sensitive than either LAT or culture tests, and highly specific. However, PCR assays have not yet become routine in clinical settings. Countercurrent immunoelectrophoresis has been shown to be an effective research diagnostic method, but has been largely supplanted by PCR.

Interaction with Streptococcus pneumoniae
Both H. influenzae and S. pneumoniae can be found in the upper respiratory system of humans. In an in vitro study of competition, S. pneumoniae always overpowered H. influenzae by attacking it with hydrogen peroxide and stripping off the surface molecules H. influenzae needs for survival.

When both bacteria are placed together into a nasal cavity, within 2 weeks, only H. influenzae survives. When either is 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 bacterium, the cells were not present.

Lab tests showed 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 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.

Treatment
Haemophilus influenzae produces beta-lactamases, and it is also able to modify its penicillin-binding proteins, so it has gained resistance to the penicillin family of antibiotics. In severe cases, cefotaxime and ceftriaxone delivered directly into the bloodstream are the elected antibiotics, and, for the less severe cases, an association of ampicillin and sulbactam, cephalosporins of the second and third generation, or fluoroquinolones are preferred. (Fluoroquinolone-resistant Haemophilus influenzae has been observed.)

Macrolide antibiotics (e.g., clarithromycin) may be used in patients with a history of allergy to beta-lactam antibiotics. Macrolide resistance has also been observed.

Prevention
Effective vaccines for Haemophilus influenzae have been available since the early 1990s, so it is preventable. Unfortunately, Hib vaccines cost about seven times the total cost of vaccines against measles, polio, tuberculosis, diphtheria, tetanus, and pertussis. Consequently, whereas 92% of the populations of developed countries was vaccinated against Hib as of 2003, vaccination coverage was 42% for developing countries, and only 8% for least-developed countries.

Sequencing
H. influenzae was the first free-living organism to have its entire genome sequenced. Completed by Craig Venter and his team, 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, and 18 other RNA genes. The sequencing method used was whole-genome shotgun, which was completed and published in Science in 1995 and conducted at The Institute for Genomic Research.