The invention relates to a recombinant Lactococcus strain, with environmentally limited growth and viability. More particularly, it relates to a recombinant Lactococcus that can only survive in a medium, where well-defined medium compounds are present. A preferred embodiment is a Lactococcus that may only survive in a host organism, where such medium compounds are present, but cannot survive outside the host organism in the absence of such medium compounds.