Whooping cough—why, he nearly whooped himself to death.
—Rosa Nouchette Carey: Uncle Max (1887)
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. H influenzae type b (Hib) produces acute, life-threatening infections of the central nervous system, epiglottis, and soft tissues, primarily in children. Disease begins with fever and lethargy, and in the case of acute meningitis, can progress to coma and death in less than 1 day. In affluent countries, Hib disease has been controlled by immunization. H influenzae also produces common but less fulminant infections of the bronchi, respiratory sinuses, and middle ear; the latter are usually associated with nonencapsulated strains. Pertussis is a prolonged illness caused by toxins produced by Bordetella pertussis bacteria attached to the cilia of respiratory epithelial cells. It progresses in stages over many weeks, beginning with rhinorrhea (runny nose), and evolving into a persistent paroxysmal cough lasting weeks more. The term “whooping cough” comes the inspiratory “whoop” made by children after an exhausting series of retching coughs. Pertussis vaccine has reduced disease incidence in developed countries, but vaccine modifications to reduce febrile seizures have led to important reductions in effectiveness.
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.
Haemophilus influenzae Gram stain. The Gram-negative bacilli are small and so short that some appear almost round. This is the basis of the term coccobacilli. The morphology of Bordetella pertussis is the same. (Reproduced with permission from Connor DH, Chandler FW, Schwartz DQ, et al: Pathology of Infectious Diseases. Stamford CT: Appleton & Lange; 1997.)
Tiny Gram-negative coccobacilli
Cultivation of Haemophilus (Greek haema, blood, and philos, loving) species requires the use of culture media enriched with blood or blood products for optimal growth. This requirement reflects bacterial 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 ...