Construction of a potential bivalent vaccine strain: introduction of Shigella sonnei form I antigen genes into the galE Salmonella typhi Ty21a typhoid vaccine strain

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Abstract
S. sonnei, an intestinal pathogen, produces a characteristic form I cell surface antigen now known to be plasmid encoded. The GalE S. typhi Ty21a oral vaccine strain, highly effective against typhoid, possibly can be modified so as to be protective also againt shigellosis due to S. sonnei. The plasmid responsible for form I antigen synthesis was therefore conjugally transferred to the galE S. typhi strain. Serological studies revealed that the derivative strain produces the form I antigen in addition to the normal S. typhi somatic antigens. Testing in mice demonstrated that the derivative form I galE S. typhi strain is evidently protective against both S. sonnei and S. typhi challenges. Thus, the galE S. typhi Ty21a oral vaccine strain, which presumably stimulates the local immune system in the intestine, may also serve as a useful carrier for other antigenic determinants to protect against different intestinal infections.