Quantifying and visualizing the transfer of exogenous plasmids to environmental microbial communities

Plasmid transfer is deemed responsible for the rapid spread of antibiotic resistance among microbes. While broad host range plasmids are known to transfer to diverse hosts in pure culture, their transfer potential to complex communities has not been comprehensively studied. The ability of a community to take up exogenous plasmid should, however, be an important element affecting the fate of mobile genetic elements released in the environment.

We have devised a method to evaluate the permissiveness of a bacterial community towards exogenous plasmids, both quantitatively (how many bacteria can take up a model plasmid?) and in term of diversity (what type of bacteria take up the plasmid?). The method takes advantage of fluorescent marker genes, image analysis, flow cytometry and next generation sequencing.

We revealed that an unexpectedly high diversity of soil microbes can take up broad host range plasmids, with common transfer across the Gram ‘barrier’. We next looked for factors that modulate permissiveness and, in particular, identified a taxon-specific effect imposed by metals when supplemented in concentrations that cause partial inhibition of the community metabolic activity.

Overall, our findings highlight the high potential for exogenous plasmids to be transferred to soil microbial communities and indicate that community permissiveness – as affected by environmental conditions- needs to be considered to predict the fate of plasmids in the environment.