ABSTRACT
Plasmids, as self-replicating and mobile genetic elements that are separate from the chromosome, generally provide accessory, but not essential functions to their hosts. In particular, traits that confer adaptations to locally restrictive conditions tend to cluster on plasmids (Eberhard, 1989). As such, and in spite of the energetic drain imposed upon host cell metabolism, plasmids can be viewed as desirable elements for host cells. They provide a mechanism for carriage of functions that can be required for survival under conditions of environmental stress but are dispensable at other times. From the plasmid’s selfish point of view, it makes sense to provide selectable functions to potential host cells, as this can enhance host competitiveness and thus host and plasmid survivability in cases of selective pressure in the environment.
