Introduction of Shigella flexneri 2a type and group antigen genes into oral typhoid vaccine strain Salmonella typhi Ty21a.

For protection against dysentery caused by Shigella flexneri 2a, an in vivo-constructed recombinant plasmid with genes specifying the S. flexneri type and group antigens located near the pro (min 6) and his (min 44) chromosomal markers, respectively, was made and transferred to the galE Salmonella typhi strain Ty21a. Strain Ty21a carrying this recombinant plasmid was shown by immunological and biochemical analyses to express the S. flexneri 2a type and group antigens. Mice immunized with this vaccine strain were found to be protected against challenge with virulent S. flexneri 2a, but not significantly against S. typhi challenge, presumably because synthesis of the Shigella antigens interfered with expression of the typhoid antigens. Elimination of the recombinant plasmid from Ty21a allowed this strain to again express typical S. typhi O antigens. Mouse protection against both S. typhi and S. flexneri 2a challenges was achieved with a whole-cell vaccine mixture composed of equal parts of Ty21a and the Ty21a-S. flexneri 2a hybrid strain.