A recA deletion mutant of Mycobacterium bovis BCG confers protection equivalent to that of wild-type BCG but shows increased genetic stability.

The widely used vaccine against tuberculosis, BCG, shows evidence of genetic instability. It has undergone major genetic rearrangements resulting in deletion and duplication of segments of its chromosome. In order to produce a BCG strain with more favourable genetic properties, we inactivated the recA gene.