Anthrax bacteria. Coloured scanning electron micrograph (SEM) of Bacillus anthracis bacteria, the cause of anthrax. These are rod-shaped, Gram-positive, spore-forming bacteria. The bacteria can infect the skin (cutaneous anthrax), causing raised itchy lesions, the lungs (pulmonary anthrax), which is fatal unless treated quickly, and the digestive system (gastrointestinal anthrax), causing vomiting of blood and severe diarrhoea. All forms can be fatal if left untreated. Treatment is with antibiotics. Magnification: x33,300 when printed at 10 centimetres tall.