Final Report Summary - COEVOCON (Coevolution of bacteria and conjugative plasmids)
Horizontal gene transfer speeds up adaptive evolution in bacteria by allowing the sharing between species of ecologically important genes, such as those encoding antibiotic resistance. This evolutionary process usually relies on mobile genetic elements that carry genes between cells, the most important being plasmids. Plasmids are semi-autonomous loops of DNA that encode genes for their own replication and transmission alongside their cargo of bacterial genes. Our research shows that acquiring a plasmid is costly for host cells, because it disrupts the normal running of the cell. These fitness costs limit the survival of plasmids in bacterial populations, but they can be ameliorated by rapid compensatory evolution to reduce the fitness cost, allowing plasmids to survive across a wide range of environmental conditions. In bacterial communities, where species vary in their ability to maintain plasmids, transfer of plasmids between species can ensure that all species retain access to the library of genes that plasmids carry. These source-to-sink dynamics of between-species plasmid transfer, from proficient plasmid host species to poor plasmid host species, can enhance the evolvability of the whole bacterial community. Furthermore, we have shown that while the benefits of the bacterial genes plasmids carry varies across environments, counter-intuitively, it is in the environments where plasmids are useless that they drive the most horizontal gene transfer between species. This is because in these environments plasmids must persist by infectious cell-to-cell transmission, which promotes the mobilization of genes from the bacterial chromosome to the plasmid. Overall, these findings help us to understand the dynamics of plasmids, and the genes they transfer, in bacterial communities. These insights could help us to predict where and when important genes, like those encoding antibiotic resistance, are most likely to be exchanged between species.