2. General Characteristics
Haemophilus is a genus of Gram-negative, pleomorphic, coccobacilli bacteria. While
Haemophilus bacteria are typically small coccobacilli, they are categorized as pleomorphic
bacteria because of the wide range of shapes they occasionally assume.
All Haemophilus are nonmotile, they are minute in size, H. influenzae measuring 0.3
micrometer across and up to 2 micrometers long. The genus includes commensal organisms
along with some significant pathogenic species such as H. influenzae a cause of sepsis and
bacterial meningitis in young children and H. ducreyi, the causative agent of chancroid.
3. Growth characteristics
• All members are either aerobic or facultative anaerobic.
• Members of the Haemophilus genus will not grow on blood agar plates as
all species require at least one of the following blood factors for growth:
hemin (factor X) and/or nicotinamide adenine dinucleotide (factor V).
• Chocolate agar is an excellent Haemophilus growth medium as it allows for
increased accessibility to these factors.
• Alternatively, Haemophilus is sometimes cultured using the "Staph streak"
technique; both Staphylococcus and Haemophilus organisms are cultured
together on a single blood agar plate.
• In this case, Haemophilus colonies will frequently grow in small "satellite"
colonies around the larger Staphylococcus colonies because the metabolism
of Staphylococcus produces the necessary blood factor by-products
required for Haemophilus growth.
4.
5. Pathogenesis and Pathology
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, reduced immune function or chronically
inflamed tissues, e.g. from allergies) create an opportunity.
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, epiglottitis and acute bacterial meningitis.
On occasion, it causes cellulitis, osteomyelitis, and infectious arthritis.
6. Vaccination
Due to routine use of the Hib conjugate vaccine in the U.S.A. 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 the vaccine is not widely
used.
Unencapsulated H. influenzae strains are unaffected by the Hib vaccine and
cause ear infections (otitis media), eye infections (conjunctivitis), and sinusitis in
children, and are associated with pneumonia.
