Salmonella bacteria. Coloured scanning electron micrograph (SEM) of many Salmonella typhimurium bacteria growing on blood agar. These bacteria are Gram-negative rods with flagellae (thread-like structures) that are used for movement. S. typhimurium is a major cause of food poisoning (salmonellosis) in humans. Salmonella bacteria are transmitted in food. Infections are common in poultry and eggs. Inadequately cooked pork and beef are also potential sources. Rats and mice are also common carriers. Clinical symptoms include headache, fever, abdominal pain and diarrhoea. Magnification: x3900 when printed 10cm wide.