Salmonella is a Gram-negative bacterium, a cylindrical rod of size about 2 microns by 0.5 microns, thus much smaller than the cells of higher organisms such as humans. In culture, the bacterium will grow on artificial media to form colonies. Salmonella Typhimurium multiplies in the gastrointestinal tract of many animal species where it usually causes no disease, but in humans its growth causes gastroenteritis. Six to 48 hours after ingestion of contaminated water or food (usually poultry or beef), illness may begin with nausea and vomiting, often followed by diarrhea. In healthy adults the disease is usually self-limiting with good medical care, but it is more serious in the young, the old, and those with underlying medical conditions; the case-fatality ratio can be as high as 5-10% in nurseries and nursing homes. Isolations of Salmonella causing gastroenteritis in humans have increased in recent years in developed countries, primarily because modern methods of animal husbandry, food preparation, and distribution encourages the spread of Salmonella. Typhimurium causes a systemic disease in mice that is similar to typhoid in humans. Thus Typhimurium has been extensively used as a model for Typhoid.
The bacterium Salmonella Typhimurium grows in the gut of almost all animals, both domesticated and wild. Humans acquire Salmonella by the ingestion of contaminated food, usually poultry products or beef, sometimes milk or water. Most isolates of Salmonella are placed in the species S. enterica, which is further subdivided into many serovars based on antigens on their surface; one of these serovars is Typhimurium. Within this serovar there is much variation in pathogenesis.
The strain LT2 was isolated in the 1940s by Lilleengen in Sweden and has been in culture since then. It is maintained in the Salmonella Genetic Stock Centre as strain SGSC1412 and in the American Type Culture Collection as strain ATCC 700720; cultures can be obtained from either source. The genome has been sequenced, and is reported in Nature 413: 852-457 (2001); the circular chromosome is 4,857 kb, and a resident plasmid, pSLT, is 94 kb.