Haemophilus

Haemophilus spp. are small, Gram-negative bacilli that can be found as part of the normal microbiota of the upper respiratory, gastrointestinal, and genital tracts. Within this genus, most infections are caused by Haemophilus influenzae. Haemophilus influenzae may or may not be encapsulated. Capsules are the basis for serogrouping Haemophilus influenzae into six groups, designated groups a through f. Nonencapsulated strains are referred to as nontypeable Haemophilus influenzae (NTHi). Capsules are a major virulence factor of this organism, since they are antiphagocytic, although NTHi strains can cause human disease. Molecular techniques have been employed to identify Haemophilus influenzae directly in clinical specimens, in particular cerebrospinal fluid, as well as Haemophilus ducreyi in suspected cases of chancroid. Identification of Haemophilus from positive blood cultures is also available as part of commercial multiplex assays.