Tuberculosis vaccine bacteria. Coloured scanning electron micrograph (SEM) of the Gram-positive rod-shaped (bacillus) Mycobacterium bovis bacteria. This is the BCG (bacillus of Calmette- Guerin) strain of the bacteria, used in the vaccination for tuberculosis (TB). The bacteria is live but attenuated (weakened) in the vaccine. The vaccine primes the immune system, without causing disease, so that it responds more rapidly if infected with TB bacteria. Magnification: x14,800 when printed 10 centimetres wide.