The ability of Listeria to survive in vivo at the early stage of infection is crucial for the induction of cell-mediated immunity [12, 13] . We examined the ability of L.inn::vgc to survive in the spleen and liver in infected mice and compared it to that of the wild type Lm. BALB/c mice were infected intravenously (i.v.) with sub-lethal doses of wild type Lm EGD-e (10 3 ), L.inn::vgc (10 7 ), or the wild type L.inn strain (10 7 ). Time points correlating with the critical phases of host immune response to listerial infection were selected and used to compare bacterial growth and induction of immune effectors in wild-type Lm, L.inn and L.inn::vgc strains. Day 3 of a Listeria infection refers to the end of the pre-immune phase before the expansion of specific T cells in the mouse model of listeriosis [14] . The presence of viable bacteria on this day has been shown to be critical for the successful induction of T cell-mediated immunity [15] . Therefore on day 3, bacterial load as well as spleen morphology was analyzed. Day 9 corresponds to the primary immune effector phase. At this time point, DTH to soluble antigen was measured in vivo as an indicator of DTH reaction and CD4+ T cell activity. Moreover, the numbers of antigen specific IFN-c producing CD8+ cytotoxic T cells were analyzed. Day 60 postinfection as well as day 5 post-challenge were chosen to analyze the memory immune effector phase [16] . To this end the number of memory effector T-cells was determined quantitatively.