Haemophilus influenzae

Haemophilus influenzae

Organism

Haemophilus influenzae is a small gram-negative bacillus.
It is nonmotile.
Facultative anaerobe (def).
Fastideous growth needs. Requires enrichments for growth.

Habitat

Mucous membranes of the respiratory tract in humans.

Source

The patient's own mucous membranes or transmitted patient-to-patient.

Epidemiology

Haemophilus parainfluenzae and nonencapsulated H. influenzae typically colonize the upper respiratory tract in humans within the first few months of life. These bacteria typically cause sinusitis, otitis media (def), bronchitis(def), and pneumonia (def).
Encapsulated H. influenzae, primarily H. influenzae type b, is uncommon as normal flora of the upper respiratory tract but can be a common cause of serious infection in children.
Until immunization of children against H. influenzae type b became routine in developed countries, this bacterium was the most common cause of pneumonia, septicemia(def), meningitis (def), and epiglottitis (def) in children under the age of four. Immunization has reduced the incidence of systemic infection by this bacterium 95%.

Clinical Disease

Haemophilus influenzae does not cause influenza. Influenza is a viral infection.
Haemophilus parainfluenzae and nonencapsulated H. influenzae typically cause sinusitis, otitis media (def), bronchitis (def), and pneumonia (def).
H. influenzae type b is the most common cause of pneumonia, septicemia (def), meningitis (def), epiglottitis (def), and cellulitis in children under the age of four who are not immunized.

Pathogenicity

In the outer membrane of the gram-negative cell wall, the lipopolysaccharide functions as an endotoxin . Endotoxin, especially when in the blood, can lead to inflammation, high fever, hypotension, capillary damage, intravascular coagulation, tissue degradation, and irreversible shock. Death is a result of what is called the shock cascade (see Fig. 1). It also impairs ciliary function leading to damage to the respiratory epithelium.
Pili (def) and a variety of cell wall adhesins (def) enable the bacterium to colonize the oropharynx and respiratory epithelium.
A capsule of polyribitol phosphate enables encapsulated strains to resist phagocytosis.
Immunoglobulin A1 proteases degrade the IgA antibodies (def) found in mucous to protect mucous membranes.

Treatment

Typically treated with broad-spectrum cephalosporins, azrithromycin, or fluroquinolones* (see antibiotic table).
Prevented by active immunization (def) with a conjugate capsular vaccine containing polyribitol phosphate capsular material from H. influenzae type b.

*Drugs may change with time.

For a more detailed article on Haemophilus influenzae infections, see Haemophilus influenzae Infections, by Mark R Schleiss, MD, Associate Professor, Department of Pediatrics, Division of Infectious Diseases, University of Cincinnati and Children's Hospital Research Foundation.