Haemophilus influenzae

The name itself is a misnomer as H.influenzae does not cause influenza. In the olden days, it was thought to be the cause of influenza and hence the name. It is a Gram negative pleomorphic, non motile, often encapsulated coccobacillus. Characteristically, it needs Factors X (hemin) and V (which technically is NAD or NADP) from blood to grow in culture. It is associated with invasive and non-invasive disease. IgA protease which breaks down IgA is an important factor for pathogenicity. Encapsulated strains are more commonly associated with invasive infections.

Meningitis especially in small children, laryngo epiglottitis or croup, pneumonia, otitis media, chronic bronchitis, sinusitis and arthritis are the common infections caused by H.influenzae.

It is a delicate microbe which needs proper care while transport and culture to isolate in culture. It dies if the samples are refrigerated. Gram stain shows typical morphology. Capsular detection can be done by the Quellung reaction where the capsule swells on mixing with antisera, latex agglutination, CIEP (counterimmunoelectrophoresis) or fluorescent antibody staining. Culture is done on chocolate agar or blood agar streaked with S.aureus which shows satellitism.