Salmonella typhimurium. Coloured scanning electron micrograph (SEM) of Salmonella typhimurium bacteria. These are Gram-negative bacilli (rod- shaped) bacteria which occur in animal hosts, including rats and mice, and are the commonest cause of food poisoning (salmonellosis) in humans. Infection occurs when these bacteria contaminate food such as meat, milk and eggs. They produce enterotoxins which act on the small intestine causing abdominal pain, nausea, vomiting and diarrhoea. The infection is treatable and is rarely fatal. Magnification x10,000 at 6x7cm size.