Multi-plasmid clash in a bacterial community: plasmid viability depends on the ecological setting of hosts

Plasmids are genetic elements that disperse horizontally between different strains and species of bacteria and a major factor in the dissemination of virulence factors and antibiotic resistance. Understanding the ecology of plasmids has a notable anthropocentric value and therefore the interactions between bacterial hosts and individual plasmids have been studied in detail. However, bacterial systems often carry multiple genetically distinct plasmids, but dynamics of these multiplasmid "clashes" has remained unstudied. Here, we set to investigate the survival of 11 mobilizable or conjugative plasmids in five different ecological settings. The key incentive was to determine whether plasmid dynamics are reproducible and whether there are trade-offs in plasmid fitness that stem from the ecological situation of their initial hosts. Growth rates and maximum population densities increased in all communities and treatments over the 42-day evolution experiment although plasmid contents at the end varied notably. We show that large multiresistance conferring plasmids are unfit when the community also contains smaller plasmids with fewer resistance genes. This suggests that restraining the use to few antibiotics can make bacterial communities sensitive to others. The hosts also appear to react to the presence of multiple genetically different plasmids by enhancing fimbriae production instead of alleviating costs of individual plasmids. In general, the survivors of the here-studied multi-plasmid clash are significantly affected by the presence or absence of antibiotic selection and plasmid-free hosts of varying fitness. Therefore, these trade-offs in different settings can explain for example why some resistance plasmids have an advantage during a rapid proliferation of antibiotic sensitive pathogen whereas others dominate in alternative situations.