Haemophilus influenzae, formerly called Pfeiffer's bacillus, is a non-motile Gram-negative
rod-shaped bacterium first described in 1892 by Richard Pfeiffer during an influenza pandemic. A member of the Pasteurellaceae family, it is generally aerobic, but can grow as a facultative anaerobe. H. influenzae was mistakenly
considered to be the cause of influenza until 1933, when the viral etiology of the flu became apparent. Still, H. influenzae is responsible for a wide range of clinical diseases.
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 or reduced immune function)
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, and acute bacterial meningitis.
Occasionally, it causes cellulitis, osteomyelitis, epiglottitis, and infectious arthritis. Due to routine use of the Hib conjugate vaccine in the U.S. 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 vaccine is not widely used. Unencapsulated H. influenzae causes ear (otitis media) and eye (conjunctivitis)infections and sinusitis in children, and is associated with pneumonia.
Bacterial culture of H. influenzae is performed on nutrient agar, preferably Chocolate agar, plate with added X(Hemin) & V(NAD) factors at 37°C in an enriched CO2 incubator.Blood agar growth is only achieved as a satellite
phenomenon around other bacteria. Colonies of H. influenzae appear as convex, smooth, pale, grey or transparent colonies. Gram-stained and microscopic observation of a specimen of H. influenzae will show Gram-negative, coccobacilli,
with no specific arrangement. Further serological is necessary to distinguish the capsular polysaccharide and differentiate between H. influenzae b and non-encapsulated species.
H. influenzae is a finicky bacterium to culture, and any modification of culture procedures can greatly reduce isolation rates. Poor quality of laboratories in developing countries has resulted in poor isolation rates of H. influenzae.
H. influenzae will grow in the hemolytic zone of Staphylococcus aureus on blood agar plates. The hemolysis of cells by S. aureus releases nutrients vital to the growth of H. influenzae. H. influenzae will not grow outside
the hemolytic zone of S. aureus due to the lack of nutrients in these areas.
Abbreviated from Wikipedia.