Construction of plasmid vectors with a non-antibiotic selection system based on the Escherichia coli thyA+ gene: application to cholera vaccine development.

The construction of live oral carriers based on attenuated Salmonella strains as vectors offers a new approach to vaccine development. We have constructed a set of plasmid vectors which have the thyA gene of Escherichia coli (encoding thymidylate synthetase) as the marker for selection and maintenance of plasmid clones. The thyA system offers an alternative to antibiotic-resistance selection markers. It can be easily adapted to a particular host-vector combination since thyA chromosomal mutations can be readily introduced by trimethoprim selection. We also describe the construction of thyA-based plasmids with the Vibrio cholerae rfb genes (encoding O-antigen biosynthesis of the Inaba serotype). These have been found to be useful in the construction of candidate bivalent cholera-typhoid vaccines.