Haemophilus influenzae

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Haemophilus influenzae and Hib Meningitis (page 1)

(This chapter has 4 pages)

© Kenneth Todar, PhD

Introduction

Haemophilus influenzae is a small, nonmotile Gram-negative bacterium in the family Pasteurellaceae. The family also includes Pasteurella and Actinobacillus, two other genera of bacteria that are parasites of animals. Encapsulated strains of Haemophilus influenzae isolated from cerebrospinal fluid are coccobacilli, 0.2 to 0.3 to 0.5 to 0.8 um, similar in morphology to Bordetella pertussis, 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 carefully carried out. Furthermore, elongated forms from sputum may exhibit bipolar staining, leading to an erroneous diagnosis of Streptococcus pneumoniae

.

Figure 1. Gram stain of Haemophilus influenzae from sputum.

H. influenzae is highly adapted to its human host. It is present in the nasopharynx of approximately 75 percent of healthy children and adults. It is rarely encountered in the oral cavity, and it has not been detected in any other animal species. It is usually the non encapsulated strains that are harbored as normal flora, but a minority of healthy individuals (3-7 percent) intermittently harbor H. influenzae type b (Hib) encapsulated strains in the upper respiratory tract. Pharyngeal carriage of Hib is important in the transmission of the bacterium. The success of current vaccination programs against Hib is due in part to the effect of vaccination on decreasing carriage of the organism.

What's in a name?

Haemophilus influenzae is widespread in its distribution among the human population. It was first isolated by Pfeiffer during the influenza pandemic of 1890. It was mistakenly thought to be the cause of the disease influenza, and it was named accordingly. Probably, H. influenzae was an important secondary invader to the influenza virus in the 1890 pandemic, as it has been during many subsequent influenza epidemics. In pigs, a synergistic association between swine influenza virus and Haemophilus suis is necessary for swine influenza. Similar situations between human influenza virus and H. influenzae have been observed in chick embryos and infant rats.

Haemophilus "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 factor (NAD or NADP). In the laboratory, it is usually grown on chocolate blood agar which is prepared by adding blood to an agar base at 80^oC. The heat releases X and V factors from the RBCs and turns the medium a chocolate brown color. The bacterium grows best at 35-37^oC and has an optimal pH of 7.6. Haemophilus influenzae is generally grown in the laboratory under aerobic conditions or under slight CO₂ tension (5% CO₂), although it is capable of glycolytic growth and of respiratory growth using nitrate as a final electron acceptor.

In 1995, Haemophilus influenzae was the first free-living organism to have its entire chromosome sequenced, sneaking in just ahead of Escherichia coli in that race, mainly because its genome is smaller in size than E. coli. For a relatively obscure bacterium, there was already a good understanding of its genetic processes, especially transformation.

Figure 2. A map of the circular chromosome of Haemophilus influenzae illustrating the location of known genes and predicted coding regions.

Observations of genetic transformation in Haemophilus have included 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 different mechanisms and is more efficient than in enteric bacteria. When developing competence, the bacterium develops membranous "blebs" in the outer membrane that contain a specific DNA-binding protein. This outer membrane protein recognizes a specific 11-base pair sequence of DNA nucleotides that appears in Haemophilus DNA with much higher frequency than in other genera 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 cloning of DNA.

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