Construction of a stable and non-resistant bivalent vaccine candidate strain against Shigella flexneri 2a and Shigella sonnei.

The E. coli cs3 gene coding for CFA/II antigen was site-specifically integrated into the asd gene locus of S. flexneri 2a vaccine strain T32, resulting in the inactivation of the asd gene. Meanwhile, the gene cluster for S. sonnei O antigen was cloned into a non-resistant expression vector pXL378 to construct the plasmid pXL390. By transforming the asd-mutant of the T32 strain with pXL390, the bivalent vaccine candidate strain FS01 against S. flexneri 2a and S. sonnei was finally obtained. Experiments showed that the recombinant plasmid pXL390 was very stable in the asd-T32 strain without the use of any antibiotics; the FS01 strain was genetically stable and expressed two kinds of Shigella LPS-O antigens on the surface without any enhancement of its toxicity. Animal tests demonstrated that FS01 strain, when administered subcutaneously in mice, could provide 100% protection against the intraperitoneal challenges of virulent S. flexneri 2a and S. sonnei.