The present invention relates generally to stabilized expression plasmid systems. The stabilized expression plasmid systems comprise an expression vector that includes a plasmid maintenance system (PMS) and, optionally, one or both of a polynucleotide encoding a selected antigen under control of a promoter, and a polynucleotide encoding a selectable marker under control of a promoter. The use of the mchI protein as a selectable marker is found in preferred embodiments of the invention.