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Whooping cough, why, he nearly whooped himself to death.
—R. N. Carey: Uncle Max
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Haemophilus and Bordetella are small, Gram-negative rods that tend to assume a coccobacillary shape. Members of both genera contain species exclusively found in humans and cause respiratory tract infections. The major species are Haemophilus influenzae, the cause of acute purulent meningitis and Bordetella pertussis, the cause of whooping cough.
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Haemophilus are among the smallest of bacteria. The curved ends of the short (1.0-1.5 μm) bacilli make many appear nearly round; hence the term coccobacilli (Figure 31–1). The cell wall has a structure similar to that of other Gram-negative bacteria. The most virulent strains of H influenzae have a polysaccharide capsule, but other species of Haemophilus are not encapsulated.
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Tiny Gram-negative coccobacilli
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The cultivation of Haemophilus species requires the use of culture media enriched with blood or blood products (Greek haema, blood, and philos, loving) for optimal growth. This requirement can be attributed to the need for exogenous hematin and/or nicotinamide adenine dinucleotide (NAD). These growth factors, also termed X factor (hematin) and V factor (NAD), are present in erythrocytes. In culture media, optimal concentrations are not available unless the red blood cells are lysed by gentle heat (chocolate agar) or added separately as a supplement. Although erythrocytes are the only convenient source of hematin, sufficient amounts of NAD may be provided by certain other bacteria and yeasts. This is responsible for the “satellite phenomenon,” in which colonies of Haemophilus have been observed to grow only in the vicinity of a colony of Staphylococcus aureus. The several species of Haemophilus are defined by their requirement for hematin and/or NAD, CO2 dependence, and other cultural characteristics (Table 31–1). Species of Haemophilus other than H influenzae have the same biology described below for the nonencapsulated strains of H influenzae.
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