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Haemophilus influenzae was first recognized in 1892 by Pfeiffer, who erroneously concluded that the bacterium was the cause of influenza. H. influenzae is a small (1- × 0.3-μm) gram-negative organism of variable shape; thus, it is often described as a pleomorphic coccobacillus. In clinical specimens such as cerebrospinal fluid (CSF) and sputum, H. influenzae frequently stains only faintly with safranin and therefore can easily be overlooked.

H. influenzae grows both aerobically and anaerobically. Its aerobic growth requires two factors: hemin (X factor) and nicotinamide adenine dinucleotide (V factor). These requirements are used in the clinical laboratory to identify the bacterium. Caution must be used to distinguish H. influenzae from Haemophilus haemolyticus, a respiratory tract commensal that has identical growth requirements. H. haemolyticus has classically been distinguished from H. influenzae by the hemolysis of the former species on horse blood agar. However, a significant proportion of isolates of H. haemolyticus have now been recognized as nonhemolytic. Analysis of various genotypic and phenotypic markers, including16S ribosomal sequences, superoxide dismutase, outer-membrane protein P6, protein D, and fuculose kinase, can be used to distinguish these two species.

Six major serotypes of H. influenzae have been identified; designated a through f, they are based on antigenically distinct polysaccharide capsules. In addition, some strains lack a polysaccharide capsule and are referred to as nontypable strains. Type b and nontypable strains are the most relevant strains clinically (Table 152-1), although encapsulated strains other than type b can cause disease. H. influenzae was the first free-living organism to have its entire genome sequenced.

TABLE 152-1Characteristics of Type b and Nontypable Strains of Haemophilus influenzae

The antigenically distinct type b capsule is a linear polymer composed of ribosyl-ribitol phosphate. Strains of H. influenzae type b (Hib) cause disease primarily in infants and children <6 years of age. Nontypable strains are primarily mucosal pathogens but occasionally cause invasive disease.


H. influenzae, an exclusively human pathogen, is spread by airborne droplets or by direct contact with secretions or fomites. Colonization with nontypable H. influenzae is a dynamic process; new strains are acquired and other strains are replaced periodically.

image The widespread use of Hib conjugate vaccines in many industrialized countries has resulted in striking decreases in the rate of nasopharyngeal ...

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