[Stability of recombinant plasmids in attenuated Salmonella typhimurium and their effects on bacterial invasiveness...

Effects of plasmid type and insertion sequence on in vitro and in vivo multiplication and invasiveness of attenuated Salmonella typhimurium were examined following transformation of the bacteria with eukaryotic expression plasmids pcDNA3 and pCI with or without heterologous gene (Newcastle disease virus F gene). Exogenous plasmids had negative impacts on the replication or invasiveness of the attenuated S. typhimurium in LB broth and/or HeLa cell monolayers as well as on its survival in live chicks. The plasmid pCI had more significant effects than pcDNA3. Introduction of heterologous gene into the plasmids not only exhibited additional negative influence on the host strain but also on their own stability therein. All these results suggest that full consideration should be given to the types of plasmids and their stability within the host strain as well as to their effects on replication and invasiveness of attenuated bacteria as the DNA vaccine delivery vector for improved immune protection.