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INTRODUCTION

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Bacteria of the genus Salmonella are highly adapted for growth in both humans and animals and cause a wide spectrum of disease. The growth of serotypes Salmonella typhi and Salmonella paratyphi is restricted to human hosts, in whom these organisms cause enteric (typhoid) fever. The remaining serotypes (nontyphoidal Salmonella, or NTS) can colonize the gastrointestinal tracts of a broad range of animals, including mammals, reptiles, birds, and insects. More than 200 serotypes of Salmonella are pathogenic to humans, in whom they often cause gastroenteritis and can be associated with localized infections and/or bacteremia.

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ETIOLOGY

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This large genus of gram-negative bacilli within the family Enterobacteriaceae consists of two species: Salmonella enterica, which contains six subspecies, and Salmonella bongori. S. enterica subspecies I includes almost all the serotypes pathogenic for humans. Members of the seven Salmonella subspecies are classified into >2500 serotypes (serovars); for simplicity, Salmonella serotypes (most of which are named for the city where they were identified) are often used as the species designation. For example, the full taxonomic designation S. enterica subspecies enterica serotype Typhimurium can be shortened to Salmonella serotype Typhimurium or simply S. typhimurium. Serotyping is based on the somatic O antigen (lipopolysaccharide cell-wall components), the surface Vi antigen (restricted to S. typhi and S. paratyphi C), and the flagellar H antigen.

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Salmonellae are gram-negative, non-spore-forming, facultatively anaerobic bacilli that measure 2–3 μm by 0.4–0.6 μm. The initial identification of salmonellae in the clinical microbiology laboratory is based on growth characteristics. Salmonellae, like other Enterobacteriaceae, produce acid on glucose fermentation, reduce nitrates, and do not produce cytochrome oxidase. In addition, all salmonellae except Salmonella gallinarum-pullorum are motile by means of peritrichous flagella, and all but S. typhi produce gas (H2S) on sugar fermentation. Notably, only 1% of clinical isolates ferment lactose; a high level of suspicion must be maintained to detect these rare clinical lactose-fermenting isolates.

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Although serotyping of all surface antigens can be used for formal identification, most laboratories perform a few simple agglutination reactions that define specific O-antigen serogroups, designated A, B, C1, C2, D, and E. Strains in these six serogroups cause ~99% of Salmonella infections in humans and other warm-blooded animals. Molecular typing methods, including pulsed-field gel electrophoresis, polymerase chain reaction fingerprinting, and genomic DNA microarray analysis, are used in epidemiologic investigations to differentiate Salmonella strains of a common serotype.

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PATHOGENESIS

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All Salmonella infections begin with ingestion of organisms, most commonly in contaminated food or water. The infectious dose ranges from 200 colony-forming units (CFU) to 106 CFU, and the ingested dose is an important determinant of incubation period and disease severity. Conditions that decrease either stomach acidity (an age of <1 year, antacid ingestion, or achlorhydric disease) or intestinal integrity (inflammatory bowel disease, prior gastrointestinal surgery, or alteration of the intestinal flora by antibiotic administration) increase susceptibility to Salmonella infection.

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Once S. typhi and S. paratyphi reach the small intestine, they penetrate the mucus layer of ...

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