Invasive plasmids as ecosystem engineers-from mechanism to application.

Horizontal gene transfer, mediated by mobile genetic elements such as conjugative plasmids, is recognised as a major driver of bacterial innovation. While predominantly explored in the context of change within individual strains and species, the broad host ranges of many plasmids mean that they can invade not just lineages but communities. This has far-reaching implications for both the fate of the plasmid and our understanding of bacterial adaptation, as well as applications for the functional engineering of microbial communities. In comparison to single-strain systems, in which plasmid invasion is largely determined by a now well-defined set of parameters-conjugation rate, fitness cost of carriage, and segregation loss-the spread of plasmids into communities is vastly more complex: governed by the wide range of dynamics within strains, but also by community dynamics, spatial heterogeneity, and the interactions between strain- and community-level selection. Here, we review the processes by which plasmids can invade communities and discuss how community complexity both constrains and facilitates plasmid spread. We further explore how this mechanistic understanding can be harnessed to enhance microbial community function.