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The family Enterobacteriaceae is a large, heterogeneous group of gram-negative bacteria. Many are normal inhabitants of the gastrointestinal tract of humans and other animals, but members also frequently cause disease in human beings.

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Among the Enterobacteriaceae, Salmonella, Shigella, Yersinia, and a number of specific phenotypes of Escherichia coli are important causes of gastroenteritis. In addition to diarrhea, these organisms cause a variety of extraintestinal infections. Each genera includes a heterogenous group of organisms that vary in their epidemiology and clinical characteristics. Enterobacteriaceae possess 3 major antigenic groups that react with antisera: (1) the O or somatic antigens, (2) the H or flagellar antigens, and (3) the K or capsular antigens. Serotyping has historically been an important means of subtyping these enteric pathogens; this technique is being partially superseded by our increasing ability to identify genotypic and phenotypic markers of virulence. This chapter discusses Salmonella, Shigella, and the diarrhea-causing E coli; Yersinia is discussed in Chapter 293.

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Salmonella are gram-negative, aerobic, nonlactose-fermenting, nonsporulating, flagellated bacilli. Salmonella are considered a single species, but are divided into approximately 2500 serotypes based on the somatic antigen (the major determinant) plus 1 or more less-strongly reacting minor somatic antigens.1 Serotyping is performed by state health department laboratories after initial isolation of the organism. Serotyping is extraordinarily useful for epidemiologic purposes, but not necessary for initial clinical management. The nomenclature of Salmonella has been simplified recently, but remains daunting. A serotype is now designated S enterica serotype Typhimurium or S enterica Enteritidis, often simplified to S enteritidis. Because several serotypes represent the majority of isolates, additional epidemiologic subtyping can be useful. Plasmid profile analysis, bacteriophage typing, restriction endonuclease analysis, ribotyping, pulsed-field gel electrophoresis, and antimicrobial susceptibility have all been used as epidemiologic tools.

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Epidemiology and Pathophysiology

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Reptiles, birds, poultry, cattle, and pigs serve as the major reservoirs for nontyphoidal Salmonella. In contrast, human beings are the only reservoir for S typhi. The primary animal reservoir varies by serotype and can serve as a clue to the source of contamination. For example, S hadar and S heidelberg are primarily associated with chickens; S enteritidis with eggs; S choleraesuis with pigs; and S marinum and S urbana with reptiles.

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In the United States, the highest incidence of nontyphoid Salmonella infection is in the first year of life—greater than 110 laboratory-confirmed cases per 100,000 population per year (eFigs. 283.1 and 283.2).3,4 Thereafter, rates of isolation decline rapidly by age 5 years and remain constant throughout adulthood. Salmonella infections show a seasonal pattern, with a consistent peak in the summer and fall. S typhi infection in the United States is uncommon (approximately 400 patients per year) and rarely occurs in children younger than 1 year of age. However, typhoid fever remains an important problem in many developing countries. In the United States, two thirds of S typhi infections are related to foreign travel. ...

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