For the first strategy, it must be considered that in today's medicine vaccines will often be administered to immunocompromised individuals and that the use of live vaccines in such subpopulations poses serious difficulties [4, 5] . The greatest disadvantage of subunit vaccines is their general requirement for strong adjuvants, as these adjuvants often induce detrimental tissue reactions. Lastly, the concept of so-called killed but metabolically active (KBMA) bacteria involves bacteria which are unable to form colonies on growth media but still have an intact protein synthesis and secretion machinery. Such mutants are reportedly capable of inducing CD4 + and CD8 + T cell responses and protection [3] . However this requires multiple injections.