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Gram-negative rods are a large group of diverse organisms (Figures 18–1, 18–2, and 19–1). In this book, these bacteria are subdivided into three clinically relevant categories, each in a separate chapter, according to whether the organism is related primarily to the enteric or the respiratory tract or to animal sources (Table 18–1). Although this approach leads to some overlap, it should be helpful because it allows general concepts to be emphasized.


Escherichia coli—Gram stain. Arrow points to a gram-negative rod. (Used with permission from Professor Shirley Lowe, University of California, San Francisco School of Medicine.)


Vibrio cholerae—Gram stain. Long arrow points to a curved gram-negative rod. Arrowhead points to a flagellum at one end of a curved gram-negative rod. (Source: Public Health Image Library, Centers for Disease Control and Prevention.)

TABLE 18–1Categories of Gram-Negative Rods

Gram-negative rods related to the enteric tract include a large number of genera. These genera have therefore been divided into three groups depending on the major anatomic location of disease, namely, (1) pathogens both within and outside the enteric tract, (2) pathogens primarily within the enteric tract, and (3) pathogens outside the enteric tract (see Table 18–1).

The frequency with which the organisms related to the enteric tract cause disease in the United States is shown in Table 18–2. Salmonella, Shigella, and Campylobacter are frequent pathogens in the gastrointestinal tract, whereas Escherichia, Vibrio, and Yersinia are less so. Enterotoxigenic strains of Escherichia coli are a common cause of diarrhea in developing countries but are less common in the United States. The medically important gram-negative rods that cause diarrhea are described in Table 18–3. Urinary tract infections are caused primarily by E. coli; the other organisms occur less commonly. The medically important gram-negative rods that cause urinary tract infections are described in Table 18–4.

TABLE 18–2Frequency of Diseases Caused in the United States by Gram-Negative Rods Related to the Enteric Tract

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