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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this technique is being partially superseded by our increasing ability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 ...