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The genus Salmonella are Gram-negative, non-spore-forming rods. Most species are motile, although a few such as S. gallinarum and S. pullorum are non-motile. There are more than 2000 Salmonella serotypes, based on cell wall "O" and flagellar "H" antigens (see Escherichia coli). The genus comprises two species: S. enterica, which is subdivided into over 2000 serovars, and S. bongori. Some serovars of S. enterica such as S. typhi cause systemic infections and typhoid fever, whereas others such as S. typhimurium cause gastroenteritis. Some serovars such as S. typhi are specialists which infect only humans, but others such as S. typhimurium are generalists which can infect humans and many other animals. Phase contrast microscopy, cells ~1 µm wide, 2-6 µm long:
S. typhi and the paratyphoid bacteria cause typhoid or typhoid-like fever in humans. Other forms of salmonellosis generally produce milder symptoms, in particular gastroenteritis involving nausea, vomiting, abdominal cramps, diarrhoea, fever, and headache. There may also be further consequences of infection - arthritis may follow 3-4 weeks after onset of the acute symptoms.
Onset of symptoms occurs 6-48 hours after ingestion of the organisms. As few as 15-20 cells may constitute an infectious dose, depending on the age and health of host, and strain differences among the members of the genus.
A large increase in foodborne salmonellosis was observed in the UK during the 1980s. This levelled off to about 30,000 reported cases per year by 1995 (but many cases of food poisoning go unreported, so this is an underestimate). The most common salmonella involved is S. enteritidis, followed by S. typhimurium. It is estimated that from 2-4 million cases of salmonellosis occur in the USA annually. Estimated costs of food-borne diseases in the USA (with salmonellosis a major component) range from 4.8 to 23 billion US dollars. The incidence of non-typhoid salmonellosis is increasing worldwide, causing millions of infections and many deaths each year.
Domestic animals act as a reservoir for the food-borne spread of host-generalist serovars such as S. typhimurium, which accounts for the high incidence of non-typhoid Salmonella infections worldwide. The organism can also survive in soil and water which has been contaminated by faeces, as well as in components of human and animal food. Pathogenic salmonellae ingested in food survive passage through the gastric acid barrier and invade the mucosa of the small and large intestine and produce toxins. Many strains are facultative intracellular parasites which produce endotoxins and enterotoxins. Invasion of epithelial cells stimulates the release of proinflammatory cytokines which induce an inflammatory reaction. The acute inflammatory response causes diarrhoea and may lead to ulceration and destruction of the mucosa. Under some circumstances, the bacteria can disseminate from the intestines to cause systemic disease.