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There are four medically important gram-negative rods typically associated with the respiratory tract, namely, Haemophilus influenzae, Bordetella pertussis, Legionella pneumophila, and Acinetobacter baumannii (Table 19–1). Hemophilus influenzae and B. pertussis are found only in humans, whereas L. pneumophila is found primarily in environmental water sources. Acinetobacter baumannii is found in environmental water sources but also colonizes the skin and upper respiratory tract.

TABLE 19–1Gram-Negative Rods Associated with the Respiratory Tract

Additional information regarding the clinical aspects of infections caused by the organisms in this chapter is provided in Part IX entitled Infectious Diseases beginning on page 593.



Hemophilus influenzae used to be the leading cause of meningitis in young children, but the use of the highly effective “conjugate” vaccine has greatly reduced the incidence of meningitis caused by this organism. It is still an important cause of upper respiratory tract infections (otitis media, sinusitis, conjunctivitis, and epiglottitis) and sepsis in children. It also causes pneumonia in adults, particularly in those with chronic obstructive lung disease. Haemophilus ducreyi, the agent of chancroid, is discussed in Chapter 27.

Important Properties

Hemophilus influenzae is a small gram-negative rod (cocco-bacillary rod)) with a polysaccharide capsule (Figure 19–1). It is one of the three important encapsulated pyogens, along with the pneumococcus and the meningococcus. Serologic typing is based on the antigenicity of the capsular polysaccharide. Of the six serotypes (a–f), type b is the most important. Type b used to cause most of the severe, invasive diseases, such as meningitis and sepsis, but the widespread use of the vaccine containing the type b capsular polysaccharide as the immunogen, has greatly reduced the incidence of invasive disease caused by this type. The type b capsule is composed of polyribitol phosphate.


Haemophilus influenzae—Gram stain. Arrows point to two small “coccobacillary” gram-negative rods. (Used with permission from Professor Shirley Lowe, University of California, San Francisco School of Medicine.)

Unencapsulated strains can also cause disease, especially mucosal diseases of the upper ...

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