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This is a group of small, gram-negative, pleomorphic bacteria that require enriched media, usually containing blood or its derivatives, for isolation. Haemophilus influenzae is a major human pathogen; Haemophilus ducreyi, a sexually transmitted pathogen, causes chancroid; six other Haemophilus species are among the normal microbiota of mucous membranes and only occasionally cause human disease. Haemophilus aphrophilus and Haemophilus paraphrophilus have been combined into a single species Aggregatibacter aphrophilus; likewise, Haemophilus segnis is now a member of the Aggregatibacter genus (Table 18-1).

TABLE 18-1Characteristics and Growth Requirements of the Haemophilus and Aggregatibacter Species Important to Humans


H influenzae is found on the mucous membranes of the upper respiratory tract in humans. It is an important cause of meningitis in unvaccinated children and causes upper and lower respiratory tract infections in children and adults.

Morphology and Identification

A. Typical Organisms

In specimens from acute infections, the organisms are short (1.5 μm) coccoid bacilli, sometimes occurring in pairs or short chains. In cultures, the morphology depends both on the length of incubation and on the medium. At 6–8 hours in rich medium, the small coccobacillary forms predominate. Later, there are longer rods and very pleomorphic forms.

Some organisms in young cultures (6–18 hours) on enriched medium express a definite capsule. The capsule is the antigen used for “typing” H influenzae (see later discussion).

B. Culture

On chocolate agar, flat, grayish, translucent colonies with diameters of 1–2 mm are present after 24 hours of incubation. IsoVitaleX in media enhances growth. H influenzae does not grow on sheep blood agar except around colonies of staphylococci (“satellite phenomenon”). Haemophilus haemolyticus and Haemophilus parahaemolyticus are hemolytic variants of H influenzae and Haemophilus parainfluenzae, respectively.

C. Growth Characteristics

Identification of organisms of the Haemophilus group depends partly on demonstrating the need for certain growth factors called X and V. Factor X acts physiologically as hemin; factor V can be replaced by nicotinamide adenine dinucleotide (NAD) or other coenzymes. Colonies of staphylococci on sheep blood agar cause the release of NAD, yielding the satellite growth phenomenon. The requirements for X and V factors of various Haemophilus species are ...

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