The Genus Haemophilus
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The Genus Haemophilus DORAN L. FINK AND JOSEPH W. ST. GEME III
Introduction All members of the genus Haemophilus are small, pleomorphic, Gram-negative coccobacilli. The genus consists of diverse species, including some that are pathogens in humans and others that are pathogens in animals. Additional species are commensal organisms and are not associated with disease. The type species is H. influenzae, which was first isolated during the 1889 influenza pandemic and was originally believed to be the causative agent of influenza.
Taxonomy Over the years, the taxonomy of the genus Haemophilus has been subject to considerable revision. At one time Haemophilus was included in the family Brucellaceae. In the eighth edition of {Bergey’s Manual of Determinative Bacteriology} (Zinnemann and Biberstein, 1974), this genus was grouped with “Genera of Uncertain Affiliation.” Since 1984, Haemophilus has been included along with Actinobacillus and Pasteurella in the family Pasteurellaceae (Kilian and Biberstein, 1984). Traditionally Haemophilus species were defined based on the requirement for factor X or factor V or both for aerobic growth. However, in the present classification scheme, some species can grow without either of these factors. Furthermore, dependence on factor X or factor V is not definitive for Haemophilus, as some species of Actinobacillus and Pasteurella also require factor V. As summarized in Table 1, H. influenzae (including biogroup aegyptius) and H. haemolyticus require both factor X and factor V, H. parainfluenzae, H. parahaemolyticus, H. paraphrohaemolyticus, H. paraphrophilus, H. segnis, H. parasuis and H. paracuniculus require factor V only, and H. ducreyi and H. haemoglobinophilus require factor X only. Haemophilus paragallinarum usually requires factor V, but factor V-independent H. paragallinarum strains also have been described and account for the
vast majority of isolates in certain geographic regions (Miflin et al., 1995). Similarly, H. aphrophilus sometimes requires factor X for primary isolation, but thereafter usually grows independently of both factor X and factor V. Haemophilus somnus and H. agni are closely related organisms that resemble the species Histophilus ovis and do not require either factor X or factor V; these organisms have not been validated as Haemophilus species and are referred to as species incertae sedis (of uncertain or doubtful affiliation). Some authors have suggested that H. somnus and H. agni be transferred to the genus Histophilus. Organisms that were formerly included in the genus Haemophilus include Actinobacillus pleuropneumoniae and Taylorella equigenitalis. Beside the requirement for factor X or factor V or both, other criteria that facilitate distinction of one Haemophilus species from another include the ability to lyse horse erythrocytes, the presence of catalase, and the pattern of sugar fermentation (Table 1). Strains of H. influenzae and H. parainfluenzae can be subdivided into biotypes based on biochemical reactions that determin
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