On the Choice of the Right Plasmid Vector(s) in the Times of Synthetic Biology.

Plasmid vectors are to this day the fundamental tools in molecular biology, but their selection is often guided by convenience rather than informed choice. This article revisits the architectural and functional features that determine plasmid performance i.e., origins of replication, copy number, cargo capacity, selection markers, and stability systems. We outline how these elements shape host range, expression dynamics, and metabolic burden, particularly as synthetic biology increasingly targets non-model bacteria. The growing need for reliable, portable vectors has driven the development of broad-host-range backbones, streamlined modular architectures such as SEVA, and alternatives to antibiotic-based selection. We also examine strategies to enhance long-term stability, including toxin-antitoxin systems and chromosomal integration via mini-transposons, recombinase-assisted platforms, and CRISPR-associated transposases. The convergence of standardization and customization, enabled by advances in DNA synthesis and emerging AI-assisted plasmid design tools is discussed also. These innovations promise flexible vector engineering tailored to diverse microbial chassis. Yet, a deeper, systems-level understanding of plasmid-host interactions will be necessary to ensure robust deployment of engineered functions in laboratory, industrial, and environmental settings.