The Haemophilus Species, the Bordetellae and Legionellae

The Haemophilus Species

This is a group of small, gram-negative, pleomorphic bacteria that require enriched media, usually containing blood or its derivatives, for isolation. Haemophilus influenzae type b is an important human pathogen; Haemophilus ducreyi, a sexually transmitted pathogen, causes chancroid; other Haemophilus species are among the normal flora of mucous membranes and only occasionally cause disease

Identification of hemin (X factor) and NAD (V factor) as growth requirements using paper disks. The top strain is only growing around the disk containing both hemin and NAD (black arrow), and is presumptively identified as H. influenzae

Haemophilus influenzae Epidemiology
Haemophilus species are typically considered to be normal flora in the upper respiratory tract of humans. They are found in the nasopharynx of approximately 75% of healthy individuals.  Normal flora H .influenzae are non-encapsulated. Less than 5% of healthy individuals harbour an encapsulated strain of Haemophilus influenzae, which serotypes as type b. Type b (Hib) has historically been considered to be the most common cause of bacterial meningitis in young children ages 3 months to 6 years.

Virulence Factors
*presence of capsule
*Non-encapsulated strains may possess a variety of cell surface proteins, fimbriae and pili which may aid in host cell attachment.