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Haemophilus influenzae and Hib Meningitis (page 1)
(This chapter has 4 pages)
© Kenneth Todar, PhD
Haemophilus influenzae is a small, nonmotile
bacterium in the family Pasteurellaceae. The family also
includes Pasteurella and Actinobacillus, two other
of bacteria that are parasites of animals. Encapsulated strains
influenzae isolated from cerebrospinal fluid are coccobacilli, 0.2
to 0.3 to 0.5 to 0.8 um, similar in morphology to Bordetella
the agent of whooping cough. Non encapsulated organisms from sputum are
pleomorphic and often exhibit long threads and filaments. The organism
may appear Gram-positive unless the Gram stain procedure is very
carried out. Furthermore, elongated forms from sputum may exhibit
staining, leading to an erroneous diagnosis of Streptococcus
Figure 1. Gram stain of Haemophilus
influenzae from sputum.
H. influenzae is highly adapted to its human host. It is
in the nasopharynx of approximately 75 percent of healthy children and
adults. It is rarely encountered in the oral cavity, and it has not
detected in any other animal species. It is usually the non
strains that are harbored as normal flora, but a minority of healthy
(3-7 percent) intermittently harbor H. influenzae type b (Hib)
strains in the upper respiratory tract. Pharyngeal carriage of Hib is
in the transmission of the bacterium. The success of current
programs against Hib is due in part to the effect of vaccination on
carriage of the organism.
What's in a name?
Haemophilus influenzae is widespread in its distribution
the human population. It was first isolated by Pfeiffer during the
pandemic of 1890. It was mistakenly thought to be the cause of the
influenza, and it was named accordingly. Probably, H. influenzae was
an important secondary invader to the influenza virus in the 1890
as it has been during many subsequent influenza epidemics. In pigs, a
association between swine influenza virus and Haemophilus suis is
necessary for swine influenza. Similar situations between human
virus and H. influenzae have been observed in chick embryos and
"loves heme", more specifically it requires a precursor
of heme in order to grow. Nutritionally, Haemophilus influenzae
prefers a complex medium and requires preformed growth factors that are
present in blood, specifically X factor (i.e., hemin) and V
(NAD or NADP). In the laboratory, it is usually grown on chocolate
agar which is prepared by adding blood to an agar base at 80oC.
heat releases X and V factors from the RBCs and turns the medium a
brown color. The bacterium grows best at 35-37oC and
pH of 7.6. Haemophilus influenzae is generally grown in the
under aerobic conditions or under slight CO2 tension (5% CO2),
although it is capable of glycolytic growth and of respiratory growth
nitrate as a final electron acceptor.
In 1995, Haemophilus influenzae was the first free-living
to have its entire chromosome sequenced, sneaking in just ahead of Escherichia
coli in that race, mainly because its genome is smaller in size
coli. For a relatively obscure bacterium, there was already a
understanding of its genetic processes, especially transformation.
Figure 2. A map of the
chromosome of Haemophilus influenzae
illustrating the location of
known genes and predicted coding regions.
Observations of genetic transformation in Haemophilus have
drug resistance and synthesis of specific capsular antigens. The latter
is thought to be the main determinant of type b H. influenzae.
Transformation in Haemophilus influenzae occurs by several
mechanisms and is more efficient than in enteric bacteria. When
competence, the bacterium develops membranous "blebs" in the outer
that contain a specific DNA-binding protein. This outer membrane
recognizes a specific 11-base pair sequence of DNA nucleotides that
in Haemophilus DNA with much higher frequency than in other
of bacteria. There is some evidence that Haemophilus is able to
undergo both interspecies and intraspecies transformation in vivo (in host
tissues). The restriction endonucleases from Haemophilus, e.g. Hind
III, are widely used in biotechnology and in the analysis and