Molecular analysis of variant plasmid forms of a bivalent Salmonella typhi-Shigella sonnei vaccine strain

The Salmonella typhi-Shigella sonnei hybrid vaccine strain 5076-IC was constructed to express the S. sonnei form I antigen, which may play an important role in producing protective immunity. Three clonal variants which existed in preparations of the vaccine could be distinguished phenotypically by lactose utilization, S. sonnei form I antigen expression, and restriction enzyme analysis of large plasmid DNA. Since expression of the form I antigen was lost in two of the clonal variants, genetic instability of the 120-MDa vaccine plasmid appeared to be a potential problem. To examine the molecular basis of this genetic instability, we hybridized large plasmid DNA isolated from the clonal variants to a variety of DNA probes encoding virulence-associated antigens and to Escherichia coli lacZ DNA. Results indicated that DNA rearrangement accompanied by deletions of plasmid material occurred in the vaccine plasmid. In addition, the vaccine plasmid did not contain some S. sonnei genetic material encoding antigenic polypeptides necessary for virulence.