Salmonella enteritidis bacteria. Coloured scanning electron micrograph (SEM) of several Salmonella enteritidis bacteria. A conjugation pilus (being used to transfer genetic material) can be seen at centre. These Gram-negative rod-shaped bacteria are naturally present in human and animal intestinal tracts. They can cause food poisoning in humans by producing toxins that act on the small intestine. Infection occurs when numbers of these bacteria are ingested in contaminated food, particularly meat, milk and chicken eggs. Symptoms include vomiting, abdominal pain and diarrhoea. The infection is controllable and is rarely fatal. Magnification: x 13,200 at 5x7cm size.