Salmonella typhimurium. Coloured transmission electron micrograph of Salmonella typhimurium bacteria dividing. The orange areas in the centre of the cells are the sites of the genetic material. This rod-shaped, Gram-negative species is a pathogen of many animals and is the commonest cause of food poisoning (Salmonellosis) in humans. It produces enterotoxins which act on the small intestine. Infection occurs when these bacteria contaminate food such as meat, milk and eggs. Symptoms of food poisoning include abdominal pain, nausea, vomiting and diarrhoea. The infection is usually controllable and is rarely fatal. Magnification x34,300 at 6x7cm size.