We then evaluated the way to deliver multiple antigen genes in separate vectors in the case that a fusion construct of multiple protective antigens is not the optimal choice when a multivalent vaccine is desired. The major challenge to achieve this goal is that the recombinant vaccine strain should stably maintain two or more expression vectors simultaneously, each carrying a unique selectable marker. To facilitate this strategy, we used a DadB+ vector to deliver the pspC gene, together with an Asd+ plasmid carrying the pspA gene to form a dual-plasmid system, which could deliver multiple antigens in a vaccine strain with Δalr ΔdadB and Δasd mutations [63]. The DadB+ plasmids are compatible with Asd+ vectors in a single vaccine strain without comprising the synthesis of individual antigens. Both plasmids are stable over 50 generations of growth, suggesting that antigen synthesis and delivery in vivo are not compromised in this system . To further reduce the possible recombination between plasmids, a recF mutation was introduced into strains . The Salmonella vaccine strain carrying both PspA and PspC by Asd+ and DadB+ vectors, respectively, induced higher serum and secretory antibody responses than the strain delivering a single antigen or a mixture of two vaccine strains each specifying one protective antigen and offered superior protection against i.p., i.v., or i.n. challenge with different serotypes of S. pneumoniae . The DadB+-Asd+ dual-plasmid system represents another important tool to develop multivalent live recombinant vaccines