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 S. typhi and S. 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 are pathogenic to humans, in whom they often cause gastroenteritis and can be associated with localized infections and/or bacteremia.
This large genus of gram-negative bacilli within the family Enterobacteriaceae consists of two species: S. enterica, which contains six subspecies, and S. bongori. S. enterica subspecies I includes almost all the serotypes pathogenic for humans. According to the current Salmonella nomenclature system, the full taxonomic designation S. enterica subspecies enterica serotype typhimurium can be shortened to Salmonella serotype typhimurium or simply S. typhimurium.
Members of the seven Salmonella subspecies are classified into >2500 serotypes (serovars) according to the somatic O antigen [lipopolysaccharide (LPS) cell-wall components], the surface Vi antigen (restricted to S. typhi and S. paratyphi C), and the flagellar H antigen. For simplicity, most Salmonella serotypes are named for the city where they were identified, and the serotype is often used as the species designation.
Salmonellae are gram-negative, non-spore-forming, facultatively anaerobic bacilli that measure 2–3 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 S. 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.
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 and polymerase chain reaction (PCR) fingerprinting, are used in epidemiologic investigations to differentiate Salmonella strains of a common serotype.
All Salmonella infections begin with ingestion of organisms, most commonly in contaminated food or water. The infectious dose is 103–106 colony-forming units. 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.
Once S. typhi and S. paratyphi reach the small intestine, they penetrate the mucus layer of the gut and traverse the intestinal layer through phagocytic microfold (M) cells that reside within Peyer's patches. Salmonellae can trigger the formation ...