The present invention provides facultatively attenuated bacterial species and methods of preparation and use thereof. The term "facultatively attenuated" as used herein refers to a bacterium which comprises a set of defined recombinant modifications which have substantially no effect on the ability of the bacterium to grow by multiplication when the bacterium is outside of its host organism, but which result in deletion of one or more genes essential for multiplication of the bacterium when the bacterium is introduced into its host organism, for example within host cells of a vaccinate recipient. These recombinant modifications take advantage of regulatory sequences which preferentially induce expression of genes within the mammalian host.
