Haemophilus influenzae is a small Gram-negative bacillus.
- It is nonmotile.
- Facultative anaerobe
- Catalase and oxidase positive
- Fastideous growth needs. Requires enrichments for growth.
- Mucous membranes of the respiratory tract in humans.
- The patient's own mucous membranes or transmitted patient-to-patient.
- 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, bronchitis, and pneumonia.
- 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, meningitis, and epiglottitis in children under the age of four. Immunization has reduced the incidence of systemic infection by this bacterium 95%.
- Haemophilus influenzae does not cause influenza. Influenza is a viral infection.
- Haemophilus parainfluenzae and nonencapsulated H. influenzae typically cause sinusitis, otitis media, bronchitis, and pneumonia.
- H. influenzae type b is the most common cause of pneumonia, septicemia, meningitis, epiglottitis, and cellulitis in children under the age of four who are not immunized.
- One of the three most common causes of bacterial meningitis in addition to Strep. pneumoniae and N. meningitidis.
- The H. influenzae type b (Hib) vaccine targets the polysaccharide capsule of H. influenzae type b
Primary Virulence Factors
- Type b polysaccharide capsule:
- most important factor in virulence
- prevents destruction by innate immune response
- can invade bloodstream directly from nasopharynx
- Fimbriae for adherence
- Neuraminidase (breaks down mucus) and IgA protease (destroys antibodies)
From 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.