Bacille Calmette-Guerin (BCG) vaccine is the only widely used live bacterial vaccine. It consists of living bacteria derived from an attenuated bovine strain of tubercle bacilli. The bacilli used for vaccine production are descendants of the original Calmette strain of BCG. The WHO has recommended the 'Danish 1331' strain for the production of BCG vaccine. Since January 1967, the BCG Laboratory at Guindy, Chennai, has been using the 'Danish 1331' strain for the production of BCG vaccine. Ref: Textbook of Preventive and Social Medicine by K Park, 19th edition, Page 160.