, 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
. 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 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.
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.
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. 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.
Clinical diagnosis of H. influenzae
is typically performed by bacterial culture or latex particle agglutination. 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 a H. influenzae
Bacterial culture of H. influenzae
is performed on nutrient agar
, preferably Chocolate agar
, plate with added X
factors at 37̊C in an enriched CO2 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
tests, both of which should be positive. Further serological is necessary to distinguish the capsular polysaccharide and differentiate between H. influenzae
b and non-encapsulated 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.
Latex particle agglutination
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.
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. Counter-current immunoelectrophoresis has been shown to be an effect 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. 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 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 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:
- When H. influenzae is attacked by S. pneumoniae, it signals the immune system to attack the S. pneumoniae
- The combination of the two species together triggers an immune system response that is not set off by either species individually.
it has been shown in cytological studies in immunodeficient patients that HI lives intracellular in epithelial cells in the epipharynx. this can inpart explain frequent reinfections with Hi in this patientgroup. (citation??)
It is unclear why H. influenzae is not affected by the immune response.