Haemophilus species
Paul Auwaerter, M.D.
MICROBIOLOGY
MICROBIOLOGY
MICROBIOLOGY
- Small aerobic Gram-negative coccobacillus [Fig] found mainly in the respiratory tract.
- Fastidious organisms, factors X (hemin, haemophilius from the Latin "blood loving") and V (nicotinamide-adenine-dinucleotide) are required for growth on chocolate agar, the preferred media.
- Haemophilus influenzae: two categories causing human colonization or infection.
- Typeable strains: six types generally recognized (Types a-f).
- Encapsulated, type B strain (Hib, with capsular antiphagocytosis and anticomplement virulence factors) historically accounts for most invasive infections and bacteremic pneumonia. Now rarely encountered due to childhood Hib vaccine success.
- H. influenzae serotype a (Hia) is increasingly noted as a cause of severe, invasive infection. Emergence due to intrinsic capsular virulence factors of Hia and pressure from the success of Hib immunization.
- Predominantly noted in U.S. and Canada. Incidence in Europe is low.
- In one series, it accounted for ~30% of invasive H. influenzae infections in Manitoba.
- Type f strain is also increasingly recognized as a cause of severe infection, though less common than Hia.
- Non-typeable strains: less invasive and cause more otitis media, epiglottitis, acute exacerbations of chronic bronchitis (AECB), sinusitis and nonbacteremic pneumonia.
- Increasingly recognized as a cause of URTIs, pneumonia and occasional bacteremia in all age groups.
- High colonization rates in children, up to 70%.
- Recent data suggest that non-typeable strains (along with Hia) account for more significant percentages of invasive Haemophilus disease[8].
- Other species: all are part of the normal flora of human oral and pharyngeal cavities but may also be causes of bacterial respiratory tract infections and endocarditis, similar to H. influenzae.
- H. parainfluenzae (most common)
- H. parahaemolyticus
- Taxonomy changes: reclassification from Haemophilus spp.[24]
- H. ducreyi, agent of chancroid, see the separate module.
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