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Haemophilus influenzae was first recognized in 1892 by Pfeiffer, who erroneously concluded that the bacterium was the cause of influenza. The bacterium is a small (1- by 0.3-μm) gram-negative organism of variable shape; hence, it is often described as a pleomorphic coccobacillus. In clinical specimens such as cerebrospinal fluid (CSF) and sputum, it 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 H. haemolyticus, a respiratory tract commensal that has identical growth requirements. H. haemolyticus has classically been distinguished from H. influenzae by hemolysis on horse blood agar. However, a significant proportion of isolates of H. haemolyticus have now been recognized as nonhemolytic. Analysis of 16S ribosomal sequences is one reliable method 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 145-1), 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 145-1 Characteristics of Type b and Nontypable Strains of Haemophilus Influenzae

The antigenically distinct type b capsule is a linear polymercomposed 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.

Epidemiology and Transmission

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.

The widespread use of Hib conjugate vaccines in many industrialized countries has resulted in striking decreases in the rate of nasopharyngeal colonization by Hib and in the incidence of Hib infection (Fig. 145-1). However, the majority of the world's children remain unimmunized. Worldwide, invasive Hib disease occurs predominantly in unimmunized children ...

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