- Small aerobic gram-negative coccobacillus found mainly in the respiratory tract.
- Haemophilus influenzae typeable strains: six types generally recognized (Types a-f).
- Encapsulated, type B strain (with capsular antiphagocytosis and anticomplement virulence factors) historically accounts for most invasive infections and bacteremic pneumonia.
- Non-typeable strains less invasive and cause more otitis media, epiglottitis, acute exacerbations of chronic bronchitis (AECB), sinusitis and nonbacteremic pneumonia. Biofilm formation may play role in upper respiratory tract infections.
- Fastidious organism; factors X (hemin, haemophilius from Latin "blood loving") and V (nicotinamide-adenine-dinucleotide) required for growth on chocolate agar.
- Other species: H. parainfluenzae (most common), H. aphrophilus, H. paraphrophilus, H. parahaemolyticus, H. segnis are all part of normal flora of oral and pharyngeal cavities but may also be causes of bacterial respiratory tract infections and endocarditis--similar to H. influenzae.
- Note: H. aphrophilus and H. segnis are now classified to new genus: Aggregatibacter aphrophilus and Aggregatibacter segnis.
- H. ducreyi, agent of Chancroid, see separate module.
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