Haemophilus

Haemophilus is a genus of Gram-negative, pleomorphic, coccobacilli bacteria.^[1] While Haemophilus bacteria are typically small coccobacilli, they are categorized as pleomorphic bacteria because of the wide range of shapes they occasionally assume. The genus includes commensal organisms along with some significant pathogenic strains such as H. influenzae—a cause of sepsis and bacterial meningitis in young children—and H. ducreyi, the causative agent of chancroid. All members are either aerobic or facultatively anaerobic.

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Haemophilus species are classified by characterization of their capsule: seven serogroups exist, a—f and e′.^[2] Capsule type b (Hib) is the most clinically significant because of its virulence.

Metabolism

Members of the Haemophilus genus are typically cultured on blood agar plates as all species require at least one of the following blood factors for growth: hemin (factor X) and/or nicotinamide adenine dinucleotide (factor V). Chocolate agar is an excellent Haemophilus growth media as it allows for increased accessibility to these factors.^[3] Alternatively, Haemophilus is sometimes cultured using the "Staph streak" technique: both Staphylococcus and Haemophilus organisms are cultured together on a single blood agar plate. In this case, Haemophilus colonies will frequently grow in small "satellite" colonies around the larger Staphylococcus colonies because the metabolism of Staphylococcus produces the necessary blood factor by-products required for Haemophilus growth.

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