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Salmonella spp.Salmonella is named in honor of the American veterinary pathologist Daniel Salmon (1850-1914), who, with his colleague Theobald Smith, first described them in 1885. They are Gram-negative, non-sporeforming, rod-shaped bacteria, usually motile and possessing a facultative but lactose(-) metabolism. The nomenclature of Salmonella strains is very confusing, with different serovars being designated as different species. To complicate matters, serovars of Salmonella can be subgrouped even further by 'phage type,' based on the specificity of phage to differentiate between extremely closely related bacteria. More than 2000 'species' have been classified. For more on Salmonella nomenclature, see J.P. Euzéby's List of Prokaryotic Names with Standing in Nomenclature. A number of Salmonella species are pathogenic, and some strains display host specificity for humans or for different livestock species. Salmonellosis is generally characterized by diarrhea, cramping, nausea, vomiting, and fever. The gastroenteritis is usually short-lived and mild, but a few strains produce a severe and often fatal disease, marked by increasing fever, headache and abdominal pain and followed in some cases by intestinal rupture, internal bleeding, shock, and death. Salmonella is usually spread by ingestion of food or water contaminated with animal feces, although some human-specific strains are spread via human fecal sources of contamination. Although most Salmonella survive for long periods in the environment, they are readily killed by acid, so a large number must be ingested for some to survive passage through the stomach. Once they reach the lower small intestine, the bacteria attach to epithelial cells via an adhesin on the bacterial surface. This contact activates a type III secretion system, and bacterial substances transferred to the host cells induce endocytosis. The bacteria multiply within the phagosome, and are released to underlying tissues. In most cases the bacteria are quickly taken up and killed by macrophages, but the body's inflammatory response causes an increase in fluid secretion, resulting in diarrhea. Some strains resist killing by macrophages and are spread throughout the body; their subsequent release by death of the macrophages leads to a systemic response. In general the strains we deal with here are different serovars of Salmonella enterica. As their name suggests Salmonella enterica are involved in causing diseases of the intestines. The three main serovars of Salmonella enterica are Typhimurium, Enteritidis, and Typhi.
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