Mouse protective capabilities of Escherichia coli hybrids expressing Salmonella typhi antigens

An Escherichia coli hybrid, F1061, expressing Salmonella typhi somatic antigens 9 and 12, and a derivative of this hybrid, E. coli hybrid WR3078, expressing the S. typhi Vi antigen in addition to somatic antigens 9 and 12, were compared with S. typhi Ty2 in experiments to test their ability, as live vaccines, to protect Swiss white mice against death from challenge with a mouse-virulent Salmonella typhimurium hybrid expressing the S. typhi antigens 9, 12, Vi, and d. When the live, vaccinating organisms were administered intraperitoneally, 87.5% of the mice immunized with S. typhi Ty2 survived challenge, as compared with 62.5% of those immunized with E. coli hybrid F1061 and 55% of those inoculated with E. coli hybrid WR3078. When live organisms were administered orally at a dose of 10(9), 67.5% of the mice immunized with S. typhi Ty2 survived challenge as compared with 47.5% of those immunized with E. coli hybrid F1061 and 40% of those administered E. coli hybrid WR3078. Thus, the protection conferred by E. coli hybrid F1061 expressing only the S. typhi somatic antigens, although significant in this system, was inferior to that conferred by S. typhi Ty2 and the addition of the S. typhi Vi antigen to this hybrid (creating E. coli hybrid WR3078) did not enhance that protection.